class TTree: public TNamed, public TAttLine, public TAttFill, public TAttMarker

 -- Default and i/o tree constructor.

 Note: We do *not* insert ourself into the current directory.

 -- Normal tree constructor.

 The tree is created in the current directory.
 Use the various functions Branch below to add branches to this tree.

 If the first character of title is a "/", the function assumes a folder name.
 In this case, it creates automatically branches following the folder hierarchy.
 splitlevel may be used in this case to control the split level.

 -- Destructor.

 -- Add a cloned tree to our list of trees to be notified whenever we change our branch addresses or when we are deleted.

 -- Add a TFriendElement to the list of friends.

 This function:
   -opens a file if filename is specified
   -reads a Tree with name treename from the file (current directory)
   -adds the Tree to the list of friends
 see other AddFriend functions

 A TFriendElement TF describes a TTree object TF in a file.
 When a TFriendElement TF is added to the the list of friends of an
 existing TTree T, any variable from TF can be referenced in a query
 to T.

   A tree keeps a list of friends. In the context of a tree (or a chain),
 friendship means unrestricted access to the friends data. In this way
 it is much like adding another branch to the tree without taking the risk
 of damaging it. To add a friend to the list, you can use the TTree::AddFriend
 method.  The tree in the diagram below has two friends (friend_tree1 and
 friend_tree2) and now has access to the variables a,b,c,i,j,k,l and m.

 The AddFriend method has two parameters, the first is the tree name and the
 second is the name of the ROOT file where the friend tree is saved.
 AddFriend automatically opens the friend file. If no file name is given,
 the tree called ft1 is assumed to be in the same file as the original tree.

 tree.AddFriend("ft1","friendfile1.root");
 If the friend tree has the same name as the original tree, you can give it
 an alias sin the context of the friendship:

 tree.AddFriend("tree1 = tree","friendfile1.root");
 Once the tree has friends, we can use TTree::Draw as if the friend's
 variables were in the original tree. To specify which tree to use in
 the Draw method, use the syntax:

 <treeName>.<branchname>.<varname>
 If the variablename is enough to uniquely identify the variable, you can
 leave out the tree and/or branch name.
 For example, these commands generate a 3-d scatter plot of variable "var"
 in the TTree tree versus variable v1 in TTree ft1 versus variable v2 in
 TTree ft2.

 tree.AddFriend("ft1","friendfile1.root");
 tree.AddFriend("ft2","friendfile2.root");
 tree.Draw("var:ft1.v1:ft2.v2");

 The picture illustrates the access of the tree and its friends with a
 Draw command.
 When AddFriend is called, the ROOT file is automatically opened and the
 friend tree (ft1) is read into memory. The new friend (ft1) is added to
 the list of friends of tree.
 The number of entries in the friend must be equal or greater to the number
 of entries of the original tree. If the friend tree has fewer entries a
 warning is given and the missing entries are not included in the histogram.
 To retrieve the list of friends from a tree use TTree::GetListOfFriends.
 When the tree is written to file (TTree::Write), the friends list is saved
 with it. And when the tree is retrieved, the trees on the friends list are
 also retrieved and the friendship restored.
 When a tree is deleted, the elements of the friend list are also deleted.
 It is possible to declare a friend tree that has the same internal
 structure (same branches and leaves) as the original tree, and compare the
 same values by specifying the tree.

  tree.Draw("var:ft1.var:ft2.var")

 -- Add a TFriendElement to the list of friends.

 The TFile is managed by the user (e.g. the user must delete the file).
 For complete description see AddFriend(const char *, const char *).
 This function:
   -reads a Tree with name treename from the file
   -adds the Tree to the list of friends

 -- Add a TFriendElement to the list of friends.

 The TTree is managed by the user (e.g., the user must delete the file).
 For a complete description see AddFriend(const char *, const char *).

 -- AutoSave tree header every fAutoSave bytes.

   When large Trees are produced, it is safe to activate the AutoSave
   procedure. Some branches may have buffers holding many entries.
   AutoSave is automatically called by TTree::Fill when the number of bytes
   generated since the previous AutoSave is greater than fAutoSave bytes.
   This function may also be invoked by the user, for example every
   N entries.
   Each AutoSave generates a new key on the file.
   Once the key with the tree header has been written, the previous cycle
   (if any) is deleted.

   Note that calling TTree::AutoSave too frequently (or similarly calling
   TTree::SetAutoSave with a small value) is an expensive operation.
   You should make tests for your own application to find a compromize
   between speed and the quantity of information you may loose in case of
   a job crash.

   In case your program crashes before closing the file holding this tree,
   the file will be automatically recovered when you will connect the file
   in UPDATE mode.
   The Tree will be recovered at the status corresponding to the last AutoSave.

   if option contains "SaveSelf", gDirectory->SaveSelf() is called.
   This allows another process to analyze the Tree while the Tree is being filled.

   By default the previous header is deleted after having written the new header.
   if option contains "Overwrite", the previous Tree header is deleted
   before written the new header. This option is slightly faster, but
   the default option is safer in case of a problem (disk quota exceeded)
   when writing the new header.

   The function returns the number of bytes written to the file.
   if the number of bytes is null, an error has occured while writing
   the header to the file.

   How to write a Tree in one process and view it from another process

   The following two scripts illustrate how to do this.
   The script treew.C is executed by process1, treer.C by process2

   ----- script treew.C
   void treew() {
     TFile f("test.root","recreate");
     TNtuple *ntuple = new TNtuple("ntuple","Demo","px:py:pz:random:i");
     Float_t px, py, pz;
     for ( Int_t i=0; i<10000000; i++) {
        gRandom->Rannor(px,py);
        pz = px*px + py*py;
        Float_t random = gRandom->Rndm(1);
        ntuple->Fill(px,py,pz,random,i);
        if (i%1000 == 1) ntuple->AutoSave("SaveSelf");
     }
   }

   ----- script treer.C
   void treer() {
      TFile f("test.root");
      TTree *ntuple = (TTree*)f.Get("ntuple");
      TCanvas c1;
      Int_t first = 0;
      while(1) {
         if (first == 0) ntuple->Draw("px>>hpx", "","",10000000,first);
         else            ntuple->Draw("px>>+hpx","","",10000000,first);
         first = (Int_t)ntuple->GetEntries();
         c1.Update();
         gSystem->Sleep(1000); //sleep 1 second
         ntuple->Refresh();
      }
   }

 -- Same as TTree::Branch() with added check that addobj matches className.

 See TTree::Branch() for other details.

 -- Same as TTree::Branch but automatic detection of the class name.
 See TTree::Branch for other details.

 -- Deprecated function. Use next function instead.

 -- Create one branch for each element in the collection.

   Each entry in the collection becomes a top level branch if the
   corresponding class is not a collection. If it is a collection, the entry
   in the collection becomes in turn top level branches, etc.
   The splitlevel is decreased by 1 everytime a new collection is found.
   For example if list is a TObjArray*
     - if splitlevel = 1, one top level branch is created for each element
        of the TObjArray.
     - if splitlevel = 2, one top level branch is created for each array element.
       if, in turn, one of the array elements is a TCollection, one top level
       branch will be created for each element of this collection.

   In case a collection element is a TClonesArray, the special Tree constructor
   for TClonesArray is called.
   The collection itself cannot be a TClonesArray.

   The function returns the total number of branches created.

   If name is given, all branch names will be prefixed with name_.

 IMPORTANT NOTE1: This function should not be called with splitlevel < 1.

 IMPORTANT NOTE2: The branches created by this function will have names
 corresponding to the collection or object names. It is important
 to give names to collections to avoid misleading branch names or
 identical branch names. By default collections have a name equal to
 the corresponding class name, eg the default name for a TList is "TList".

 Example--------------------------------------------------------------:

{
TTree T("T","test list");
TList *l = new TList();

TObjArray *a1 = new TObjArray();
a1->SetName("a1");
l->Add(a1);
TH1F *ha1a = new TH1F("ha1a","ha1",100,0,1);
TH1F *ha1b = new TH1F("ha1b","ha1",100,0,1);
a1->Add(ha1a);
a1->Add(ha1b);
TObjArray *b1 = new TObjArray();
b1->SetName("b1");
l->Add(b1);
TH1F *hb1a = new TH1F("hb1a","hb1",100,0,1);
TH1F *hb1b = new TH1F("hb1b","hb1",100,0,1);
b1->Add(hb1a);
b1->Add(hb1b);

TObjArray *a2 = new TObjArray();
a2->SetName("a2");
l->Add(a2);
TH1S *ha2a = new TH1S("ha2a","ha2",100,0,1);
TH1S *ha2b = new TH1S("ha2b","ha2",100,0,1);
a2->Add(ha2a);
a2->Add(ha2b);

T.Branch(l,16000,2);
T.Print();
}

 -- Create one branch for each element in the folder.
 Returns the total number of branches created.

 -- Create a new TTree Branch.

     This Branch constructor is provided to support non-objects in
     a Tree. The variables described in leaflist may be simple variables
     or structures.
    See the two following constructors for writing objects in a Tree.

    By default the branch buffers are stored in the same file as the Tree.
    use TBranch::SetFile to specify a different file

       * address is the address of the first item of a structure
         or the address of a pointer to an object (see example).
       * leaflist is the concatenation of all the variable names and types
         separated by a colon character :
         The variable name and the variable type are separated by a slash (/).
         The variable type may be 0,1 or 2 characters. If no type is given,
         the type of the variable is assumed to be the same as the previous
         variable. If the first variable does not have a type, it is assumed
         of type F by default. The list of currently supported types is given below:
            - C : a character string terminated by the 0 character
            - B : an 8 bit signed integer (Char_t)
            - b : an 8 bit unsigned integer (UChar_t)
            - S : a 16 bit signed integer (Short_t)
            - s : a 16 bit unsigned integer (UShort_t)
            - I : a 32 bit signed integer (Int_t)
            - i : a 32 bit unsigned integer (UInt_t)
            - F : a 32 bit floating point (Float_t)
            - D : a 64 bit floating point (Double_t)
            - L : a 64 bit signed integer (Long64_t)
            - l : a 64 bit unsigned integer (ULong64_t)
            - O : a boolean (Bool_t)

         By default, a variable will be copied to the buffer with the number of
         bytes specified in the type descriptor character. However, if the type
         consists of 2 characters, the second character is an integer that
         specifies the number of bytes to be used when copying the variable
         to the output buffer. Example:
             X         ; variable X, type Float_t
             Y/I       : variable Y, type Int_t
             Y/I2      ; variable Y, type Int_t converted to a 16 bits integer


       * bufsize is the buffer size in bytes for this branch
         The default value is 32000 bytes and should be ok for most cases.
         You can specify a larger value (eg 256000) if your Tree is not split
         and each entry is large (Megabytes)
         A small value for bufsize is optimum if you intend to access
         the entries in the Tree randomly and your Tree is in split mode.

 create a new branch with the object of class classname at address addobj.

 WARNING:
 Starting with Root version 3.01, the Branch function uses the new style
 branches (TBranchElement). To get the old behaviour, you can:
   - call BranchOld or
   - call TTree::SetBranchStyle(0)

 Note that with the new style, classname does not need to derive from TObject.
 It must derived from TObject if the branch style has been set to 0 (old)

 Use splitlevel < 0 instead of splitlevel=0 when the class
 has a custom Streamer

 Note: if the split level is set to the default (99),  TTree::Branch will
 not issue a warning if the class can not be split.

 -- Create a new TTree BranchObject.

    Build a TBranchObject for an object of class classname.
    addobj is the address of a pointer to an object of class classname.
    IMPORTANT: classname must derive from TObject.
    The class dictionary must be available (ClassDef in class header).

    This option requires access to the library where the corresponding class
    is defined. Accessing one single data member in the object implies
    reading the full object.
    See the next Branch constructor for a more efficient storage
    in case the entry consists of arrays of identical objects.

    By default the branch buffers are stored in the same file as the Tree.
    use TBranch::SetFile to specify a different file

      IMPORTANT NOTE about branch names
    In case two or more master branches contain subbranches with
    identical names, one must add a "." (dot) character at the end
    of the master branch name. This will force the name of the subbranch
    to be master.subbranch instead of simply subbranch.
    This situation happens when the top level object (say event)
    has two or more members referencing the same class.
    For example, if a Tree has two branches B1 and B2 corresponding
    to objects of the same class MyClass, one can do:
       tree.Branch("B1.","MyClass",&b1,8000,1);
       tree.Branch("B2.","MyClass",&b2,8000,1);
    if MyClass has 3 members a,b,c, the two instructions above will generate
    subbranches called B1.a, B1.b ,B1.c, B2.a, B2.b, B2.c

    bufsize is the buffer size in bytes for this branch
    The default value is 32000 bytes and should be ok for most cases.
    You can specify a larger value (eg 256000) if your Tree is not split
    and each entry is large (Megabytes)
    A small value for bufsize is optimum if you intend to access
    the entries in the Tree randomly and your Tree is in split mode.

 -- Build the optional branch supporting the TRefTable.
 This branch will keep all the information to find the branches
 containing referenced objects.

 At each Tree::Fill, the branch numbers containing the
 referenced objects are saved to the TBranchRef basket.
 When the Tree header is saved (via TTree::Write), the branch
 is saved keeping the information with the pointers to the branches
 having referenced objects.

 -- Create a new TTree BranchElement.

    WARNING about this new function

    This function is designed to replace the function TTree::Branch above.
    This function is far more powerful than the Branch function.
    It supports the full C++, including STL and has the same behaviour
    in split or non-split mode. classname does not have to derive from TObject.
    The function is based on the new TStreamerInfo.

    Build a TBranchElement for an object of class classname.
    addobj is the address of a pointer to an object of class classname.
    The class dictionary must be available (ClassDef in class header).

    This option requires access to the library where the corresponding class
    is defined. Accessing one single data member in the object implies
    reading the full object.

    By default the branch buffers are stored in the same file as the Tree.
    use TBranch::SetFile to specify a different file

      IMPORTANT NOTE about branch names
    In case two or more master branches contain subbranches with
    identical names, one must add a "." (dot) character at the end
    of the master branch name. This will force the name of the subbranch
    to be master.subbranch instead of simply subbranch.
    This situation happens when the top level object (say event)
    has two or more members referencing the same class.
    For example, if a Tree has two branches B1 and B2 corresponding
    to objects of the same class MyClass, one can do:
       tree.Branch("B1.","MyClass",&b1,8000,1);
       tree.Branch("B2.","MyClass",&b2,8000,1);
    if MyClass has 3 members a,b,c, the two instructions above will generate
    subbranches called B1.a, B1.b ,B1.c, B2.a, B2.b, B2.c

    bufsize is the buffer size in bytes for this branch
    The default value is 32000 bytes and should be ok for most cases.
    You can specify a larger value (eg 256000) if your Tree is not split
    and each entry is large (Megabytes)
    A small value for bufsize is optimum if you intend to access
    the entries in the Tree randomly and your Tree is in split mode.

    Use splitlevel < 0 instead of splitlevel=0 when the class
    has a custom Streamer

    Note: if the split level is set to the default (99),  TTree::Branch will
    not issue a warning if the class can not be split.

 -- Browse content of the TTree.

 -- Build a Tree Index (default is TTreeIndex).
 See a description of the parameters and functionality in
 TTreeIndex::TTreeIndex().

 The return value is the number of entries in the Index (< 0 indicates failure).

 A TTreeIndex object pointed by fTreeIndex is created.
 This object will be automatically deleted by the TTree destructor.
 See also comments in TTree::SetTreeIndex().

 -- Build StreamerInfo for class cl.
 pointer is an optional argument that may contain a pointer to an object of cl.

 -- Called by TTree::Fill() when file has reached its maximum fgMaxTreeSize.
 Create a new file. If the original file is named "myfile.root",
 subsequent files are named "myfile_1.root", "myfile_2.root", etc.

 Returns a pointer to the new file.

 Currently, the automatic change of file is restricted
 to the case where the tree is in the top level directory.
 The file should not contain sub-directories.

 Before switching to a new file, the tree header is written
 to the current file, then the current file is closed.

 To process the multiple files created by ChangeFile, one must use
 a TChain.

 The new file name has a suffix "_N" where N is equal to fFileNumber+1.
 By default a Root session starts with fFileNumber=0. One can set
 fFileNumber to a different value via TTree::SetFileNumber.
 In case a file named "_N" already exists, the function will try
 a file named "__N", then "___N", etc.

 fgMaxTreeSize can be set via the static function TTree::SetMaxTreeSize.
 The default value of fgMaxTreeSize is 1.9 Gigabytes.

 If the current file contains other objects like TH1 and TTree,
 these objects are automatically moved to the new file.

 IMPORTANT NOTE:
 Be careful when writing the final Tree header to the file!
 Don't do:
  TFile *file = new TFile("myfile.root","recreate");
  TTree *T = new TTree("T","title");
  T->Fill(); //loop
  file->Write();
  file->Close();
 but do the following:
  TFile *file = new TFile("myfile.root","recreate");
  TTree *T = new TTree("T","title");
  T->Fill(); //loop
  file = T->GetCurrentFile(); //to get the pointer to the current file
  file->Write();
  file->Close();

 -- Check whether or not the address described by the last 3 parameters matches the content of the branch.

 -- Create a clone of this tree and copy nentries.

 By default copy all entries.
 Note that only active branches are copied.
 The compression level of the cloned tree is set to the destination file's
 compression level.

 IMPORTANT: The cloned tree stays connected with this tree until this tree
            is deleted.  In particular, any changes in branch addresses
            in this tree are forwarded to the clone trees, unless a branch
            in a clone tree has had its address changed, in which case
            that change stays in effect.  When this tree is deleted, all the
            addresses of the cloned tree are reset to their default values.

 If 'option' contains the word 'fast' and nentries is -1 and no branch
 is disabled, the clone will be done without unzipping or unstreaming
 the baskets (i.e., a direct copy of the raw bytes on disk).

 When 'fast' is specified, 'option' can also contains a
 sorting order for the baskets in the output file.

 There is currently 3 supported sorting order:
    SortBasketsByOffset (the default)
    SortBasketsByBranch
    SortBasketsByEntry

 When using SortBasketsByOffset the baskets are written in
 the output file in the same order as in the original file
 (i.e. the basket are sorted on their offset in the original
 file; Usually this also means that the baskets are sorted
 on the index/number of the _last_ entry they contain)

 When using SortBasketsByBranch all the baskets of each
 individual branches are stored contiguously.  This tends to
 optimize reading speed when reading a small number (1->5) of
 branches, since all their baskets will be clustered together
 instead of being spread across the file.  However it might
 decrease the performance when reading more branches (or the full
 entry).

 When using SortBasketsByEntry the baskets with the lowest
 starting entry are written first.  (i.e. the baskets are
 sorted on the index/number of the first entry they contain).
 This means that on the file the baskets will be in the order
 in which they will be needed when reading the whole tree
 sequentially.

 For examples of CloneTree, see tutorials:

  -- copytree

     A macro to copy a subset of a TTree to a new TTree.

     The input file has been generated by the program in $ROOTSYS/test/Event
     with:  Event 1000 1 1 1

  -- copytree2

     A macro to copy a subset of a TTree to a new TTree.

     One branch of the new Tree is written to a separate file.

     The input file has been generated by the program in $ROOTSYS/test/Event
     with:  Event 1000 1 1 1

 -- Set branch addresses of passed tree equal to ours.
 -- If undo is true, reset the branch address instead of copying them.
    This insures 'separation' of a cloned tree from its original

 -- Copy nentries from given tree to this tree.

 By default copy all entries.

 Returns number of bytes copied to this tree.

 -- Copy a tree with selection.

 IMPORTANT:

   The returned copied tree stays connected with the original tree
   until the original tree is deleted.  In particular, any changes
   to the branch addresses in the original tree are also made to
   the copied tree.  Any changes made to the branch addresses of the
   copied tree are overridden anytime the original tree changes its
   branch addresses.  When the original tree is deleted, all the
   branch addresses of the copied tree are set to zero.

 For examples of CopyTree, see the tutorials:

 -- copytree

 Example macro to copy a subset of a tree to a new tree.

 The input file was generated by running the program in
 $ROOTSYS/test/Event in this way:

      ./Event 1000 1 1 1

 -- copytree2

 Example macro to copy a subset of a tree to a new tree.

 One branch of the new tree is written to a separate file.

 The input file was generated by running the program in
 $ROOTSYS/test/Event in this way:

      ./Event 1000 1 1 1

 -- copytree3

 Example macro to copy a subset of a tree to a new tree.

 Only selected entries are copied to the new tree.
 NOTE that only the active branches are copied.

 -- Create a basket for this tree and given branch.

 -- Delete this tree from memory or/and disk.

  if option == "all" delete Tree object from memory AND from disk
                     all baskets on disk are deleted. All keys with same name
                     are deleted.
  if option =="" only Tree object in memory is deleted.

 Draw expression varexp for specified entries.
 Returns -1 in case of error or number of selected events in case of success.

      This function accepts TCut objects as arguments.
      Useful to use the string operator +
         example:
            ntuple.Draw("x",cut1+cut2+cut3);

 Draw expression varexp for specified entries.
 Returns -1 in case of error or number of selected events in case of success.

  varexp is an expression of the general form
   - "e1"           produces a 1-d histogram (TH1F) of expression "e1"
   - "e1:e2"        produces an unbinned 2-d scatter-plot (TGraph) of "e1" versus "e2"
   - "e1:e2:e3"     produces an unbinned 3-d scatter-plot (TPolyMarker3D) of "e1"
                    versus "e2" versus "e3"
   - "e1:e2:e3:e4"  produces an unbinned 3-d scatter-plot (TPolyMarker3D) of "e1"
                    versus "e2" versus "e3" and "e4" mapped on the color number.
  (to create histograms in the 2, 3, and 4 dimesional case, see section "Saving
  the result of Draw to an histogram")

  Example:
     varexp = x     simplest case: draw a 1-Dim distribution of column named x
            = sqrt(x)            : draw distribution of sqrt(x)
            = x*y/z
            = y:sqrt(x) 2-Dim distribution of y versus sqrt(x)
            = px:py:pz:2.5*E  produces a 3-d scatter-plot of px vs py ps pz
              and the color number of each marker will be 2.5*E.
              If the color number is negative it is set to 0.
              If the color number is greater than the current number of colors
                 it is set to the highest color number.
              The default number of colors is 50.
              see TStyle::SetPalette for setting a new color palette.

  Note that the variables e1, e2 or e3 may contain a selection.
  example, if e1= x*(y<0), the value histogrammed will be x if y<0
  and will be 0 otherwise.

  The expressions can use all the operations and build-in functions
  supported by TFormula (See TFormula::Analyze), including free
  standing function taking numerical arguments (TMath::Bessel).
  In addition, you can call member functions taking numerical
  arguments. For example:
      - "TMath::BreitWigner(fPx,3,2)"
      - "event.GetHistogram().GetXaxis().GetXmax()"
      - "event.GetTrack(fMax).GetPx()

  selection is an expression with a combination of the columns.
  In a selection all the C++ operators are authorized.
  The value corresponding to the selection expression is used as a weight
  to fill the histogram.
  If the expression includes only boolean operations, the result
  is 0 or 1. If the result is 0, the histogram is not filled.
  In general, the expression may be of the form:
      value*(boolean expression)
  if boolean expression is true, the histogram is filled with
  a weight = value.
  Examples:
      selection1 = "x<y && sqrt(z)>3.2"
      selection2 = "(x+y)*(sqrt(z)>3.2)"
  selection1 returns a weigth = 0 or 1
  selection2 returns a weight = x+y if sqrt(z)>3.2
             returns a weight = 0 otherwise.

  option is the drawing option.
    - See TH1::Draw for the list of all drawing options.
    - If option COL is specified when varexp has three fields:
            tree.Draw("e1:e2:e3","","col");
      a 2D scatter is produced with e1 vs e2, and e3 is mapped on the color
      table.
    - If option contains the string "goff", no graphics is generated.

  nentries is the number of entries to process (default is all)
  first is the first entry to process (default is 0)

  This function returns the number of selected entries. It returns -1
  if an error occurs.

     Drawing expressions using arrays and array elements

 Let assumes, a leaf fMatrix, on the branch fEvent, which is a 3 by 3 array,
 or a TClonesArray.
 In a TTree::Draw expression you can now access fMatrix using the following
 syntaxes:

   String passed    What is used for each entry of the tree

   "fMatrix"       the 9 elements of fMatrix
   "fMatrix[][]"   the 9 elements of fMatrix
   "fMatrix[2][2]" only the elements fMatrix[2][2]
   "fMatrix[1]"    the 3 elements fMatrix[1][0], fMatrix[1][1] and fMatrix[1][2]
   "fMatrix[1][]"  the 3 elements fMatrix[1][0], fMatrix[1][1] and fMatrix[1][2]
   "fMatrix[][0]"  the 3 elements fMatrix[0][0], fMatrix[1][0] and fMatrix[2][0]

   "fEvent.fMatrix...." same as "fMatrix..." (unless there is more than one leaf named fMatrix!).

 In summary, if a specific index is not specified for a dimension, TTree::Draw
 will loop through all the indices along this dimension.  Leaving off the
 last (right most) dimension of specifying then with the two characters '[]'
 is equivalent.  For variable size arrays (and TClonesArray) the range
 of the first dimension is recalculated for each entry of the tree.

 TTree::Draw also now properly handling operations involving 2 or more arrays.

 Let assume a second matrix fResults[5][2], here are a sample of some
 of the possible combinations, the number of elements they produce and
 the loop used:

  expression                       element(s)  Loop

  "fMatrix[2][1] - fResults[5][2]"   one     no loop
  "fMatrix[2][]  - fResults[5][2]"   three   on 2nd dim fMatrix
  "fMatrix[2][]  - fResults[5][]"    two     on both 2nd dimensions
  "fMatrix[][2]  - fResults[][1]"    three   on both 1st dimensions
  "fMatrix[][2]  - fResults[][]"     six     on both 1st and 2nd dimensions of
                                             fResults
  "fMatrix[][2]  - fResults[3][]"    two     on 1st dim of fMatrix and 2nd of
                                             fResults (at the same time)
  "fMatrix[][]   - fResults[][]"     six     on 1st dim then on  2nd dim


 In summary, TTree::Draw loops through all un-specified dimensions.  To
 figure out the range of each loop, we match each unspecified dimension
 from left to right (ignoring ALL dimensions for which an index has been
 specified), in the equivalent loop matched dimensions use the same index
 and are restricted to the smallest range (of only the matched dimensions).
 When involving variable arrays, the range can of course be different
 for each entry of the tree.

 So the loop equivalent to "fMatrix[][2] - fResults[3][]" is:

    for (Int_t i0; i < min(3,2); i++) {
       use the value of (fMatrix[i0][2] - fMatrix[3][i0])
    }

 So the loop equivalent to "fMatrix[][2] - fResults[][]" is:

    for (Int_t i0; i < min(3,5); i++) {
       for (Int_t i1; i1 < 2; i1++) {
          use the value of (fMatrix[i0][2] - fMatrix[i0][i1])
       }
    }

 So the loop equivalent to "fMatrix[][] - fResults[][]" is:

    for (Int_t i0; i < min(3,5); i++) {
       for (Int_t i1; i1 < min(3,2); i1++) {
          use the value of (fMatrix[i0][i1] - fMatrix[i0][i1])
       }
    }

     Retrieving the result of Draw


  By default the temporary histogram created is called "htemp", but only in
  the one dimensional Draw("e1") it contains the TTree's data points. For
  a two dimensional Draw, the data is filled into a TGraph which is named
  "Graph". They can be retrieved by calling
    TH1F *htemp = (TH1F*)gPad->GetPrimitive("htemp"); // 1D
    TGraph *graph = (TGraph*)gPad->GetPrimitive("Graph"); // 2D

  For a three and four dimensional Draw the TPloyMarker3D is unnamed, and
  cannot be retrieved.

  gPad always contains a TH1 derived object called "htemp" which allows to
  access the axes:
    TGraph *graph = (TGraph*)gPad->GetPrimitive("Graph"); // 2D
    TH2F   *htemp = (TH2F*)gPad->GetPrimitive("htemp"); // empty, but has axes
    TAxis  *xaxis = htemp->GetXaxis();

     Saving the result of Draw to an histogram


  If varexp0 contains >>hnew (following the variable(s) name(s),
  the new histogram created is called hnew and it is kept in the current
  directory (and also the current pad). This works for all dimensions.
  Example:
    tree.Draw("sqrt(x)>>hsqrt","y>0")
    will draw sqrt(x) and save the histogram as "hsqrt" in the current
    directory.  To retrieve it do:
    TH1F *hsqrt = (TH1F*)gDirectory->Get("hsqrt");

  The binning information is taken from the environment variables

     Hist.Binning.?D.?

  In addition, the name of the histogram can be followed by up to 9
  numbers between '(' and ')', where the numbers describe the
  following:

   1 - bins in x-direction
   2 - lower limit in x-direction
   3 - upper limit in x-direction
   4-6 same for y-direction
   7-9 same for z-direction

   When a new binning is used the new value will become the default.
   Values can be skipped.
  Example:
    tree.Draw("sqrt(x)>>hsqrt(500,10,20)")
          // plot sqrt(x) between 10 and 20 using 500 bins
    tree.Draw("sqrt(x):sin(y)>>hsqrt(100,10,60,50,.1,.5)")
          // plot sqrt(x) against sin(y)
          // 100 bins in x-direction; lower limit on x-axis is 10; upper limit is 60
          //  50 bins in y-direction; lower limit on y-axis is .1; upper limit is .5

  By default, the specified histogram is reset.
  To continue to append data to an existing histogram, use "+" in front
  of the histogram name.
  A '+' in front of the histogram name is ignored, when the name is followed by
  binning information as described in the previous paragraph.
    tree.Draw("sqrt(x)>>+hsqrt","y>0")
      will not reset hsqrt, but will continue filling.
  This works for 1-D, 2-D and 3-D histograms.

     Accessing collection objects


  TTree::Draw default's handling of collections is to assume that any
  request on a collection pertain to it content.  For example, if fTracks
  is a collection of Track objects, the following:
      tree->Draw("event.fTracks.fPx");
  will plot the value of fPx for each Track objects inside the collection.
  Also
     tree->Draw("event.fTracks.size()");
  would plot the result of the member function Track::size() for each
  Track object inside the collection.
  To access information about the collection itself, TTree::Draw support
  the '@' notation.  If a variable which points to a collection is prefixed
  or postfixed with '@', the next part of the expression will pertain to
  the collection object.  For example:
     tree->Draw("event.@fTracks.size()");
  will plot the size of the collection refered to by fTracks (i.e the number
  of Track objects).

     Drawing 'objects'


  When a class has a member function named AsDouble or AsString, requesting
  to directly draw the object will imply a call to one of the 2 functions.
  If both AsDouble and AsString are present, AsDouble will be used.
  AsString can return either a char*, a std::string or a TString.s
  For example, the following
     tree->Draw("event.myTTimeStamp");
  will draw the same histogram as
     tree->Draw("event.myTTimeStamp.AsDouble()");
  In addition, when the object is a type TString or std::string, TTree::Draw
  will call respectively TString::Data and std::string::c_str()

  If the object is a TBits, the histogram will contain the index of the bit
  that are turned on.

     Retrieving  information about the tree itself.


  You can refer to the tree (or chain) containing the data by using the
  string 'This'.
  You can then could any TTree methods.  For example:
     tree->Draw("This->GetReadEntry()");
  will display the local entry numbers be read.
     tree->Draw("This->GetUserInfo()->At(0)->GetName()");
  will display the name of the first 'user info' object.

     Special functions and variables


  Entry$:  A TTree::Draw formula can use the special variable Entry$
  to access the entry number being read.  For example to draw every
  other entry use:
    tree.Draw("myvar","Entry$%2==0");

  Entry$      : return the current entry number (== TTree::GetReadEntry())
  LocalEntry$ : return the current entry number in the current tree of a chain (== GetTree()->GetReadEntry())
  Entries$    : return the total number of entries (== TTree::GetEntries())
  Length$     : return the total number of element of this formula for this
                 entry (==TTreeFormula::GetNdata())
  Iteration$: return the current iteration over this formula for this
                 entry (i.e. varies from 0 to Length$).

  Length$(formula): return the total number of element of the formula given as a
                    parameter.
  Sum$(formula): return the sum of the value of the elements of the formula given
                    as a parameter.  For eaxmple the mean for all the elements in
                    one entry can be calculated with:
                Sum$(formula)/Length$(formula)

  Alt$(primary,alternate) : return the value of "primary" if it is available
                 for the current iteration otherwise return the value of "alternate".
                 For example, with arr1[3] and arr2[2]
    tree->Draw("arr1+Alt$(arr2,0)");
                 will draw arr1[0]+arr2[0] ; arr1[1]+arr2[1] and arr1[2]+0
                 Or with a variable size array arr3
    tree->Draw("Alt$(arr3[0],0)+Alt$(arr3[1],0)+Alt$(arr3[2],0)");
                 will draw the sum arr3 for the index 0 to min(2,actual_size_of_arr3-1)
                 As a comparison
    tree->Draw("arr3[0]+arr3[1]+arr3[2]");
                 will draw the sum arr3 for the index 0 to 2 only if the
                 actual_size_of_arr3 is greater or equal to 3.
                 Note that the array in 'primary' is flatened/linearilized thus using
                 Alt$ with multi-dimensional arrays of different dimensions in unlikely
                 to yield the expected results.  To visualize a bit more what elements
                 would be matched by TTree::Draw, TTree::Scan can be used:
    tree->Scan("arr1:Alt$(arr2,0)");
                 will print on one line the value of arr1 and (arr2,0) that will be
                 matched by
    tree->Draw("arr1-Alt$(arr2,0)");

     Drawing a user function accessing the TTree data directly


  If the formula contains  a file name, TTree::MakeProxy will be used
  to load and execute this file.   In particular it will draw the
  result of a function with the same name as the file.  The function
  will be executed in a context where the name of the branches can
  be used as a C++ variable.

  For example draw px using the file hsimple.root (generated by the
  hsimple.C tutorial), we need a file named hsimple.cxx:

     double hsimple() {
        return px;
     }

  MakeProxy can then be used indirectly via the TTree::Draw interface
  as follow:
     new TFile("hsimple.root")
     ntuple->Draw("hsimple.cxx");

  A more complete example is available in the tutorials directory:
    h1analysisProxy.cxx , h1analysProxy.h and h1analysisProxyCut.C
  which reimplement the selector found in h1analysis.C

  The main features of this facility are:

    * on-demand loading of branches
    * ability to use the 'branchname' as if it was a data member
    * protection against array out-of-bound
    * ability to use the branch data as object (when the user code is available)

  See TTree::MakeProxy for more details.

     Making a Profile histogram

  In case of a 2-Dim expression, one can generate a TProfile histogram
  instead of a TH2F histogram by specyfying option=prof or option=profs.
  The option=prof is automatically selected in case of y:x>>pf
  where pf is an existing TProfile histogram.

     Making a parallel coordinates plot.

  In case of a 2-Dim or more expression with the option=para, one can generate
  a parallel coordinates plot. With that option, the number of dimensions is
  arbitrary. Giving more than 4 variables without the option=para or
  option=candle or option=goff will produce an error.

     Making a candle sticks chart.

  In case of a 2-Dim or more expression with the option=candle, one can generate
  a candle sticks chart. With that option, the number of dimensions is
  arbitrary. Giving more than 4 variables without the option=para or
  option=candle or option=goff will produce an error.

     Saving the result of Draw to a TEventList or a TEntryList

  TTree::Draw can be used to fill a TEventList object (list of entry numbers)
  instead of histogramming one variable.
  If varexp0 has the form >>elist , a TEventList object named "elist"
  is created in the current directory. elist will contain the list
  of entry numbers satisfying the current selection.
  If option "entrylist" is used, a TEntryList object is created
  Example:
    tree.Draw(">>yplus","y>0")
    will create a TEventList object named "yplus" in the current directory.
    In an interactive session, one can type (after TTree::Draw)
       yplus.Print("all")
    to print the list of entry numbers in the list.
    tree.Draw(">>yplus", "y>0", "entrylist")
    will create a TEntryList object names "yplus" in the current directory

  By default, the specified entry list is reset.
  To continue to append data to an existing list, use "+" in front
  of the list name;
    tree.Draw(">>+yplus","y>0")
      will not reset yplus, but will enter the selected entries at the end
      of the existing list.

      Using a TEventList or a TEntryList as Input

  Once a TEventList or a TEntryList object has been generated, it can be used as input
  for TTree::Draw. Use TTree::SetEventList or TTree::SetEntryList to set the
  current event list
  Example1:
     TEventList *elist = (TEventList*)gDirectory->Get("yplus");
     tree->SetEventList(elist);
     tree->Draw("py");
  Example2:
     TEntryList *elist = (TEntryList*)gDirectory->Get("yplus");
     tree->SetEntryList(elist);
     tree->Draw("py");
  If a TEventList object is used as input, a new TEntryList object is created
  inside the SetEventList function. In case of a TChain, all tree headers are loaded
  for this transformation. This new object is owned by the chain and is deleted
  with it, unless the user extracts it by calling GetEntryList() function.
  See also comments to SetEventList() function of TTree and TChain.

  If arrays are used in the selection critera, the entry entered in the
  list are all the entries that have at least one element of the array that
  satisfy the selection.
  Example:
      tree.Draw(">>pyplus","fTracks.fPy>0");
      tree->SetEventList(pyplus);
      tree->Draw("fTracks.fPy");
  will draw the fPy of ALL tracks in event with at least one track with
  a positive fPy.

  To select only the elements that did match the original selection
  use TEventList::SetReapplyCut or TEntryList::SetReapplyCut.
  Example:
      tree.Draw(">>pyplus","fTracks.fPy>0");
      pyplus->SetReapplyCut(kTRUE);
      tree->SetEventList(pyplus);
      tree->Draw("fTracks.fPy");
  will draw the fPy of only the tracks that have a positive fPy.

  Note: Use tree->SetEventList(0) if you do not want use the list as input.

      How to obtain more info from TTree::Draw


  Once TTree::Draw has been called, it is possible to access useful
  information still stored in the TTree object via the following functions:
    -GetSelectedRows()    // return the number of entries accepted by the
                          //selection expression. In case where no selection
                          //was specified, returns the number of entries processed.
    -GetV1()              //returns a pointer to the double array of V1
    -GetV2()              //returns a pointer to the double array of V2
    -GetV3()              //returns a pointer to the double array of V3
    -GetW()               //returns a pointer to the double array of Weights
                          //where weight equal the result of the selection expression.
   where V1,V2,V3 correspond to the expressions in
   TTree::Draw("V1:V2:V3",selection);

   Example:
    Root > ntuple->Draw("py:px","pz>4");
    Root > TGraph *gr = new TGraph(ntuple->GetSelectedRows(),
                                   ntuple->GetV2(), ntuple->GetV1());
    Root > gr->Draw("ap"); //draw graph in current pad
    creates a TGraph object with a number of points corresponding to the
    number of entries selected by the expression "pz>4", the x points of the graph
    being the px values of the Tree and the y points the py values.

   Important note: By default TTree::Draw creates the arrays obtained
    with GetV1, GetV2, GetV3, GetW with a length corresponding to the
    parameter fEstimate. By default fEstimate=10000 and can be modified
    via TTree::SetEstimate. A possible recipee is to do
       tree->SetEstimate(tree->GetEntries());
    You must call SetEstimate if the expected number of selected rows
    is greater than 10000.

    You can use the option "goff" to turn off the graphics output
    of TTree::Draw in the above example.

           Automatic interface to TTree::Draw via the TTreeViewer


    A complete graphical interface to this function is implemented
    in the class TTreeViewer.
    To start the TTreeViewer, three possibilities:
       - select TTree context menu item "StartViewer"
       - type the command  "TTreeViewer TV(treeName)"
       - execute statement "tree->StartViewer();"

 -- Remove some baskets from memory.

 -- Drop branch buffers to accomodate nbytes below MaxVirtualsize.

 -- Fill all branches.

   This function loops on all the branches of this tree.  For
   each branch, it copies to the branch buffer (basket) the current
   values of the leaves data types. If a leaf is a simple data type,
   a simple conversion to a machine independent format has to be done.

   The function returns the number of bytes committed to the
   individual branches.

   If a write error occurs, the number of bytes returned is -1.

   If no data are written, because, e.g., the branch is disabled,
   the number of bytes returned is 0.

 -- FIXME: Describe this function.

 -- FIXME: Describe this function.

 -- Fit  a projected item(s) from a tree.

  funcname is a TF1 function.

  See TTree::Draw() for explanations of the other parameters.

  By default the temporary histogram created is called htemp.
  If varexp contains >>hnew , the new histogram created is called hnew
  and it is kept in the current directory.

  The function returns the number of selected entries.

  Example:
    tree.Fit(pol4,sqrt(x)>>hsqrt,y>0)
    will fit sqrt(x) and save the histogram as "hsqrt" in the current
    directory.

   See also TTree::UnbinnedFit

   Return status

  The function returns the status of the histogram fit (see TH1::Fit)
  If no entries were selected, the function returns -1;
   (ie fitResult is null is the fit is OK)

 -- Returns the expanded value of the alias.  Search in the friends if any.

 -- Return pointer to the branch with the given name in this tree or its friends.

 -- Return status of branch with name branchname.
 0 if branch is not activated
 1 if branch is activated

 -- Static function returning the current branch style.
 style = 0 old Branch
 style = 1 new Bronch

 -- Return pointer to the current file.

 Return the number of entries matching the selection.
 Return -1 in case of errors.

 If the selection uses any arrays or containers, we return the number
 of entries where at least one element match the selection.
 GetEntries is implemented using the selector class TSelectorEntries,
 which can be used directly (see code in TTreePlayer::GetEntries) for
 additional option.
 If SetEventList was used on the TTree or TChain, only that subset
 of entries will be considered.

 -- Return number of entries of this tree if not zero, otherwise return the number of entries in the first friend tree.

 -- Read all branches of entry and return total number of bytes read.

     getall = 0 : get only active branches
     getall = 1 : get all branches

  The function returns the number of bytes read from the input buffer.
  If entry does not exist  the function returns 0.
  If an I/O error occurs,  the function returns -1.

  If the Tree has friends, also read the friends entry

  To activate/deactivate one or more branches, use TBranch::SetBranchStatus
  For example, if you have a Tree with several hundred branches, and you
  are interested only by branches named "u" and "v", do
     mytree.SetBranchStatus("*",0); //disable all branches
     mytree.SetBranchStatus("a",1);
     mytree.SetBranchStatus("b",1);
  when calling mytree.GetEntry(i); only branches "a" and "b" will be read.

  WARNING!!
  If your Tree has been created in split mode with a parent branch "parent",
     mytree.SetBranchStatus("parent",1);
  will not activate the sub-branches of "parent". You should do:
     mytree.SetBranchStatus("parent*",1);

  An alternative is to call directly
     brancha.GetEntry(i)
     branchb.GetEntry(i);

  IMPORTANT NOTE

 By default, GetEntry reuses the space allocated by the previous object
 for each branch. You can force the previous object to be automatically
 deleted if you call mybranch.SetAutoDelete(kTRUE) (default is kFALSE).
 Example:
 Consider the example in $ROOTSYS/test/Event.h
 The top level branch in the tree T is declared with:
    Event *event = 0;  //event must be null or point to a valid object
                       //it must be initialized
    T.SetBranchAddress("event",&event);
 When reading the Tree, one can choose one of these 3 options:

   OPTION 1


    for (Long64_t i=0;i<nentries;i++) {
       T.GetEntry(i);
       // the object event has been filled at this point
    }
   The default (recommended). At the first entry an object of the
   class Event will be created and pointed by event.
   At the following entries, event will be overwritten by the new data.
   All internal members that are TObject* are automatically deleted.
   It is important that these members be in a valid state when GetEntry
   is called. Pointers must be correctly initialized.
   However these internal members will not be deleted if the characters "->"
   are specified as the first characters in the comment field of the data
   member declaration.
   If "->" is specified, the pointer member is read via pointer->Streamer(buf).
   In this case, it is assumed that the pointer is never null (case
   of pointer TClonesArray *fTracks in the Event example).
   If "->" is not specified, the pointer member is read via buf >> pointer.
   In this case the pointer may be null. Note that the option with "->"
   is faster to read or write and it also consumes less space in the file.

   OPTION 2

  The option AutoDelete is set
   TBranch *branch = T.GetBranch("event");
   branch->SetAddress(&event);
   branch->SetAutoDelete(kTRUE);
    for (Long64_t i=0;i<nentries;i++) {
       T.GetEntry(i);
       // the object event has been filled at this point
    }
   In this case, at each iteration, the object event is deleted by GetEntry
   and a new instance of Event is created and filled.

   OPTION 3

   Same as option 1, but you delete yourself the event.
    for (Long64_t i=0;i<nentries;i++) {
       delete event;
       event = 0;  // EXTREMELY IMPORTANT
       T.GetEntry(i);
       // the object event has been filled at this point
    }

  It is strongly recommended to use the default option 1. It has the
  additional advantage that functions like TTree::Draw (internally
  calling TTree::GetEntry) will be functional even when the classes in the
  file are not available.

Returns the entry list, set to this tree

The returned object is not owned by the tree, even if the entry list was created
by the SetEventList() function (see also comments of SetEventList())

 -- Return entry number corresponding to entry.

 if no TEntryList set returns entry
 else returns the entry number corresponding to the list index=entry

 -- Return entry number corresponding to major and minor number.
 Note that this function returns only the entry number, not the data
 To read the data corresponding to an entry number, use TTree::GetEntryWithIndex
 the BuildIndex function has created a table of Long64_t* of sorted values
 corresponding to val = major<<31 + minor;
 The function performs binary search in this sorted table.
 If it finds a pair that maches val, it returns directly the
 index in the table.
 If an entry corresponding to major and minor is not found, the function
 returns the index of the major,minor pair immediatly lower than the
 requested value, ie it will return -1 if the pair is lower than
 the first entry in the index.

 See also GetEntryNumberWithIndex

 -- Return entry number corresponding to major and minor number.
 Note that this function returns only the entry number, not the data
 To read the data corresponding to an entry number, use TTree::GetEntryWithIndex
 the BuildIndex function has created a table of Long64_t* of sorted values
 corresponding to val = major<<31 + minor;
 The function performs binary search in this sorted table.
 If it finds a pair that maches val, it returns directly the
 index in the table, otherwise it returns -1.

 See also GetEntryNumberWithBestIndex

 -- Read entry corresponding to major and minor number.

  The function returns the total number of bytes read.
  If the Tree has friend trees, the corresponding entry with
  the index values (major,minor) is read. Note that the master Tree
  and its friend may have different entry serial numbers corresponding
  to (major,minor).

 Return a pointer to the TTree friend whose name or alias is 'friendname.

 -- If the the 'tree' is a friend, this method returns its alias name.

 This alias is an alternate name for the tree.

 It can be used in conjunction with a branch or leaf name in a TTreeFormula,
 to specify in which particular tree the branch or leaf can be found if
 the friend trees have branches or leaves with the same name as the master
 tree.

 It can also be used in conjunction with an alias created using
 TTree::SetAlias in a TTreeFormula, e.g.:

      maintree->Draw("treealias.fPx - treealias.myAlias");

 where fPx is a branch of the friend tree aliased as 'treealias' and 'myAlias'
 was created using TTree::SetAlias on the friend tree.

 However, note that 'treealias.myAlias' will be expanded literally,
 without remembering that it comes from the aliased friend and thus
 the branch name might not be disambiguated properly, which means
 that you may not be able to take advantage of this feature.

 -- Creates a new iterator that will go through all the leaves on the tree itself and its friend.

 -- Return pointer to the 1st Leaf named name in any Branch of this Tree or any branch in the list of friend trees.

  aname may be of the form branchname/leafname

 -- Return maximum of column with name columname.

 -- Static function which returns the tree file size limit.

 -- Return minimum of column with name columname.

 -- Return name corresponding to colindex in varexp.

   varexp is a string of names separated by :
   index is an array with pointers to the start of name[i] in varexp

 -- Load the TTreePlayer (if not already done).

 -- Return a pointer to the list containing user objects associated to this tree.

 The list is automatically created if it does not exist.

 WARNING: By default the TTree destructor will delete all objects added
          to this list. If you do not want these objects to be deleted,
          call:

               mytree->GetUserInfo()->Clear();

          before deleting the tree.

 -- Keep a maximum of fMaxEntries in memory.

 -- Read in memory all baskets from all branches up to the limit of maxmemory bytes.

 If maxmemory is non null and positive SetMaxVirtualSize is called
 with this value. Default for maxmemory is 2000000000 (2 Gigabytes).
 The function returns the total number of baskets read into memory
 if negative an error occured while loading the branches.
 This method may be called to force branch baskets in memory
 when random access to branch entries is required.
 If random access to only a few branches is required, you should
 call directly TBranch::LoadBaskets.

 -- Set current entry.

 Returns -2 if entry does not exist (just as TChain::LoadTree()).

 Note: This function is overloaded in TChain.

 -- Load entry on behalf of our master tree, we may use an index.

 Called by LoadTree() when the masterTree looks for the entry
 number in a friend tree (us) corresponding to the passed entry
 number in the masterTree.

 If we have no index, our entry number and the masterTree entry
 number are the same.

 If we *do* have an index, we must find the (major, minor) value pair
 in masterTree to locate our corresponding entry.

 -- Generate a skeleton analysis class for this tree.

 The following files are produced: classname.h and classname.C.
 If classname is 0, classname will be called "nameoftree".

 The generated code in classname.h includes the following:
    - Identification of the original tree and the input file name.
    - Definition of an analysis class (data members and member functions).
    - The following member functions:
       - constructor (by default opening the tree file),
       - GetEntry(Long64_t entry),
       - Init(TTree* tree) to initialize a new TTree,
       - Show(Long64_t entry) to read and dump entry.

 The generated code in classname.C includes only the main
 analysis function Loop.

 To use this function:
    - Open your tree file (eg: TFile f("myfile.root");)
    - T->MakeClass("MyClass");
 where T is the name of the TTree in file myfile.root,
 and MyClass.h, MyClass.C the name of the files created by this function.
 In a ROOT session, you can do:
    root > .L MyClass.C
    root > MyClass* t = new MyClass;
    root > t->GetEntry(12); // Fill data members of t with entry number 12.
    root > t->Show();       // Show values of entry 12.
    root > t->Show(16);     // Read and show values of entry 16.
    root > t->Loop();       // Loop on all entries.

  NOTE: Do not use the code generated for a single TTree which is part
        of a TChain to process that entire TChain.  The maximum dimensions
        calculated for arrays on the basis of a single TTree from the TChain
        might be (will be!) too small when processing all of the TTrees in
        the TChain.  You must use myChain.MakeClass() to generate the code,
        not myTree.MakeClass(...).

 -- Generate a skeleton function for this tree.

 The function code is written on filename.
 If filename is 0, filename will be called nameoftree.C

 The generated code includes the following:
    - Identification of the original Tree and Input file name,
    - Opening the Tree file,
    - Declaration of Tree variables,
    - Setting of branches addresses,
    - A skeleton for the entry loop.

 To use this function:
    - Open your Tree file (eg: TFile f("myfile.root");)
    - T->MakeCode("MyAnalysis.C");
 where T is the name of the TTree in file myfile.root
 and MyAnalysis.C the name of the file created by this function.

 NOTE: Since the implementation of this function, a new and better
       function TTree::MakeClass() has been developed.

 -- Build index array for names in varexp.

 -- Generate a skeleton analysis class for this Tree using TBranchProxy.

 TBranchProxy is the base of a class hierarchy implementing an
 indirect access to the content of the branches of a TTree.

 "proxyClassname" is expected to be of the form:
    [path/]fileprefix
 The skeleton will then be generated in the file:
    fileprefix.h
 located in the current directory or in 'path/' if it is specified.
 The class generated will be named 'fileprefix'

 "macrofilename" and optionally "cutfilename" are expected to point
 to source files which will be included by the generated skeleton.
 Method of the same name as the file(minus the extension and path)
 will be called by the generated skeleton's Process method as follow:
    [if (cutfilename())] htemp->Fill(macrofilename());

 "option" can be used select some of the optional features during
 the code generation.  The possible options are:
    nohist : indicates that the generated ProcessFill should not
             fill the histogram.

 'maxUnrolling' controls how deep in the class hierachy does the
 system 'unroll' classes that are not split.  Unrolling a class
 allows direct access to its data members (this emulates the behavior
 of TTreeFormula).

 The main features of this skeleton are:

    * on-demand loading of branches
    * ability to use the 'branchname' as if it was a data member
    * protection against array out-of-bounds errors
    * ability to use the branch data as an object (when the user code is available)

 For example with Event.root, if
    Double_t somePx = fTracks.fPx[2];
 is executed by one of the method of the skeleton,
 somePx will updated with the current value of fPx of the 3rd track.

 Both macrofilename and the optional cutfilename are expected to be
 the name of source files which contain at least a free standing
 function with the signature:
     x_t macrofilename(); // i.e function with the same name as the file
 and
     y_t cutfilename();   // i.e function with the same name as the file

 x_t and y_t needs to be types that can convert respectively to a double
 and a bool (because the skeleton uses:
     if (cutfilename()) htemp->Fill(macrofilename());

 These two functions are run in a context such that the branch names are
 available as local variables of the correct (read-only) type.

 Note that if you use the same 'variable' twice, it is more efficient
 to 'cache' the value. For example
   Int_t n = fEventNumber; // Read fEventNumber
   if (n<10 || n>10) { ... }
 is more efficient than
   if (fEventNumber<10 || fEventNumber>10)

 Also, optionally, the generated selector will also call methods named
 macrofilename_methodname in each of 6 main selector methods if the method
 macrofilename_methodname exist (Where macrofilename is stripped of its
 extension).

 Concretely, with the script named h1analysisProxy.C,

 The method         calls the method (if it exist)
 Begin           -> h1analysisProxy_Begin
 SlaveBegin      -> h1analysisProxy_SlaveBegin
 Notify          -> h1analysisProxy_Notify
 Process         -> h1analysisProxy_Process
 SlaveTerminate  -> h1analysisProxy_SlaveTerminate
 Terminate       -> h1analysisProxy_Terminate

 If a file name macrofilename.h (or .hh, .hpp, .hxx, .hPP, .hXX) exist
 it is included before the declaration of the proxy class.  This can
 be used in particular to insure that the include files needed by
 the macro file are properly loaded.

 The default histogram is accessible via the variable named 'htemp'.

 If the library of the classes describing the data in the branch is
 loaded, the skeleton will add the needed #include statements and
 give the ability to access the object stored in the branches.

 To draw px using the file hsimple.root (generated by the
 hsimple.C tutorial), we need a file named hsimple.cxx:

     double hsimple() {
        return px;
     }

 MakeProxy can then be used indirectly via the TTree::Draw interface
 as follow:
     new TFile("hsimple.root")
     ntuple->Draw("hsimple.cxx");

 A more complete example is available in the tutorials directory:
   h1analysisProxy.cxx , h1analysProxy.h and h1analysisProxyCut.C
 which reimplement the selector found in h1analysis.C

 -- Generate skeleton selector class for this tree.

 The following files are produced: selector.h and selector.C.
 If selector is 0, the selector will be called "nameoftree".

 The generated code in selector.h includes the following:
    - Identification of the original Tree and Input file name
    - Definition of selector class (data and functions)
    - The following class functions:
       - constructor and destructor
       - void    Begin(TTree *tree)
       - void    Init(TTree *tree)
       - Bool_t  Notify()
       - Bool_t  Process(Long64_t entry)
       - void    Terminate

 The class selector derives from TSelector.
 The generated code in selector.C includes empty functions defined above:

 To use this function:
    - connect your Tree file (eg: TFile f("myfile.root");)
    - T->MakeSelector("myselect");
 where T is the name of the Tree in file myfile.root
 and myselect.h, myselect.C the name of the files created by this function.
 In a ROOT session, you can do:
    root > T->Process("myselect.C")

 -- Check if adding nbytes to memory we are still below MaxVirtualsize.

 -- Static function merging the trees in the TList into a new tree.

 Trees in the list can be memory or disk-resident trees.
 The new tree is created in the current directory (memory if gROOT).

 -- Merge the trees in the TList into this tree.

 Returns the total number of entries in the merged tree.

 -- Function called when loading a new class library.

 -- Interface to the Principal Components Analysis class.

   Create an instance of TPrincipal
   Fill it with the selected variables
   if option "n" is specified, the TPrincipal object is filled with
                 normalized variables.
   If option "p" is specified, compute the principal components
   If option "p" and "d" print results of analysis
   If option "p" and "h" generate standard histograms
   If option "p" and "c" generate code of conversion functions
   return a pointer to the TPrincipal object. It is the user responsability
   to delete this object.
   The option default value is "np"

   see TTree::Draw for explanation of the other parameters.

   The created object is  named "principal" and a reference to it
   is added to the list of specials Root objects.
   you can retrieve a pointer to the created object via:
      TPrincipal *principal =
        (TPrincipal*)gROOT->GetListOfSpecials()->FindObject("principal");

 -- Print a summary of the tree contents.

 If option contains "all" friend trees are also printed.
 If option contains "toponly" only the top level branches are printed.

 Wildcarding can be used to print only a subset of the branches, e.g.,
 T.Print("Elec*") will print all branches with name starting with "Elec".

 Process this tree executing the code in filename.
 The return value is -1 in case of error and TSelector::GetStatus() in
 in case of success.

 The code in filename is loaded (interpreted or compiled , see below)
 filename must contain a valid class implementation derived from TSelector.
 where TSelector has the following member functions:

     void TSelector::Begin(). This function is called before looping on the
          events in the Tree. The user can create his histograms in this function.

     Bool_t TSelector::ProcessCut(Long64_t entry). This function is called
          before processing entry. It is the user's responsability to read
          the corresponding entry in memory (may be just a partial read).
          The function returns kTRUE if the entry must be processed,
          kFALSE otherwise.
     void TSelector::ProcessFill(Long64_t entry). This function is called for
          all selected events. User fills histograms in this function.
     void TSelector::Terminate(). This function is called at the end of
          the loop on all events.
     void TTreeProcess::Begin(). This function is called before looping on the
          events in the Tree. The user can create his histograms in this function.

   if filename is of the form file.C, the file will be interpreted.
   if filename is of the form file.C++, the file file.C will be compiled
      and dynamically loaded.
   if filename is of the form file.C+, the file file.C will be compiled
      and dynamically loaded. At next call, if file.C is older than file.o
      and file.so, the file.C is not compiled, only file.so is loaded.

   The function returns the number of processed entries. It returns -1
   in case of an error.

  NOTE1
  It may be more interesting to invoke directly the other Process function
  accepting a TSelector* as argument.eg
     MySelector *selector = (MySelector*)TSelector::GetSelector(filename);
     selector->CallSomeFunction(..);
     mytree.Process(selector,..);

  NOTE2
  One should not call this function twice with the same selector file
  in the same script. If this is required, proceed as indicated in NOTE1,
  by getting a pointer to the corresponding TSelector,eg
    workaround 1

void stubs1() {
   TSelector *selector = TSelector::GetSelector("h1test.C");
   TFile *f1 = new TFile("stubs_nood_le1.root");
   TTree *h1 = (TTree*)f1->Get("h1");
   h1->Process(selector);
   TFile *f2 = new TFile("stubs_nood_le1_coarse.root");
   TTree *h2 = (TTree*)f2->Get("h1");
   h2->Process(selector);
}
  or use ACLIC to compile the selector
   workaround 2

void stubs2() {
   TFile *f1 = new TFile("stubs_nood_le1.root");
   TTree *h1 = (TTree*)f1->Get("h1");
   h1->Process("h1test.C+");
   TFile *f2 = new TFile("stubs_nood_le1_coarse.root");
   TTree *h2 = (TTree*)f2->Get("h1");
   h2->Process("h1test.C+");
}

 Process this tree executing the code in selector.
 The return value is -1 in case of error and TSelector::GetStatus() in
 in case of success.

 The TSelector class has the following member functions:

   void TSelector::Begin(). This function is called before looping on the
        events in the Tree. The user can create his histograms in this function.

   Bool_t TSelector::ProcessCut(Long64_t entry). This function is called
        before processing entry. It is the user's responsability to read
        the corresponding entry in memory (may be just a partial read).
        The function returns kTRUE if the entry must be processed,
        kFALSE otherwise.
   void TSelector::ProcessFill(Long64_t entry). This function is called for
        all selected events. User fills histograms in this function.
   void TSelector::Terminate(). This function is called at the end of
        the loop on all events.
   void TTreeProcess::Begin(). This function is called before looping on the
        events in the Tree. The user can create his histograms in this function.

 -- Make a projection of a tree using selections.

 Depending on the value of varexp (described in Draw) a 1-D, 2-D, etc.,
 projection of the tree will be filled in histogram hname.
 Note that the dimension of hname must match with the dimension of varexp.

 -- Loop over entries and return a TSQLResult object containing entries following selection.

 -- Create or simply read branches from filename.

 if branchDescriptor = "" (default), it is assumed that the Tree descriptor
    is given in the first line of the file with a syntax like
     A/D:Table[2]/F:Ntracks/I:astring/C
  otherwise branchDescriptor must be specified with the above syntax.
  -If the type of the first variable is not specified, it is assumed to be "/F"
  -if the type of any other variable is not specified, the type of the previous
    variable is assumed. eg
      x:y:z      (all variables are assumed of type "F"
      x/D:y:z    (all variables are of type "D"
      x:y/D:z    (x is type "F", y and z of type "D"
  -If the type is a string of characters. This will read
  subsequent characters until a whitespace is found (whitespace
  characters are considered to be blank, newline and tab).

 Lines in the input file starting with "#" are ignored.
 This function will read and ignore any whitespace characters
 (this includes blank spaces and the newline and tab characters).

 A TBranch object is created for each variable in the expression.
 The total number of rows read from the file is returned.

  -- Refresh contents of this tree and its branches from the current status on disk.

  One can call this function in case the tree file is being
  updated by another process.

 -- Remove a friend from the list of friends.

 -- Reset baskets, buffers and entries count in all branches and leaves.

 -- Tell the branch to drop their current objects and allocate new ones
    or to return to the default address (TNtuple and TNtupleD)

 -- Tell all of our branches to drop their current objects and allocate new ones.

 -- Loop over tree entries and print entries passing selection.

 If varexp is 0 (or "") then print only first 8 columns.
 If varexp = "*" print all columns.
 Otherwise a columns selection can be made using "var1:var2:var3".
 See TTreePlayer::Scan for more information

 -- Set a tree variable alias.

  Set an alias for an expression/formula based on the tree 'variables'.

  The content of 'aliasName' can be used in TTreeFormula (i.e. TTree::Draw,
  TTree::Scan, TTreeViewer) and will be evaluated as the content of
  'aliasFormula'.
  If the content of 'aliasFormula' only contains symbol names, periods and
  array index specification (for example event.fTracks[3]), then
  the content of 'aliasName' can be used as the start of symbol.

  If the alias 'aliasName' already existed, it is replaced by the new
  value.

  When being used, the alias can be preceded by an eventual 'Friend Alias'
  (see TTree::GetFriendAlias)

  Return true if it was added properly.

  For example:
     tree->SetAlias("x1","(tdc1[1]-tdc1[0])/49");
     tree->SetAlias("y1","(tdc1[3]-tdc1[2])/47");
     tree->SetAlias("x2","(tdc2[1]-tdc2[0])/49");
     tree->SetAlias("y2","(tdc2[3]-tdc2[2])/47");
     tree->Draw("y2-y1:x2-x1");

     tree->SetAlias("theGoodTrack","event.fTracks[3]");
     tree->Draw("theGoodTrack.fPx"); // same as "event.fTracks[3].fPx"

 -- Set a branch's basket size.

 bname is the name of a branch.
 if bname="*", apply to all branches.
 if bname="xxx*", apply to all branches with name starting with xxx
 see TRegexp for wildcarding options
 buffsize = branc basket size

 -- Change branch address, dealing with clone trees properly.

 -- Verify the validity of the type of add before calling SetBranchAddress.

 -- Verify the validity of the type of add before calling SetBranchAddress.

 -- Set branch status to Process or DoNotProcess.

  When reading a Tree, by default, all branches are read.
  One can speed up considerably the analysis phase by activating
  only the branches that hold variables involved in a query.

     bname is the name of a branch.
     if bname="*", apply to all branches.
     if bname="xxx*", apply to all branches with name starting with xxx
     see TRegexp for wildcarding options
      status = 1  branch will be processed
             = 0  branch will not be processed
    Example:
  Assume a tree T with sub-branches a,b,c,d,e,f,g,etc..
  when doing T.GetEntry(i) all branches are read for entry i.
  to read only the branches c and e, one can do
    T.SetBranchStatus("*",0); //disable all branches
    T.SetBranchStatus("c",1);
    T.setBranchStatus("e",1);
    T.GetEntry(i);

  WARNING! WARNING! WARNING!
  SetBranchStatus is matching the branch based on regular expression match
  of the branch 'name' and not on the branch hierarchy!
  In order to be able to selectively enable a top level object that is 'split'
  you need to make sure the name of the top level branch is prefixed to the
  sub-branches' name(by adding a dot ('.') at the end of the Branch creation
  and use the corresponding regular expression.

  I.e If your Tree has been created in split mode with a parent branch "parent."
  (note the trailing dot).
     T.SetBranchStatus("parent",1);
  will not activate the sub-branches of "parent". You should do:
     T.SetBranchStatus("parent*",1);

  Without the trailing dot in the branch creation you have no choice but to
  call SetBranchStatus explicitly for each of the sub branches.


  An alternative to this function is to read directly and only
  the interesting branches. Example:
    TBranch *brc = T.GetBranch("c");
    TBranch *bre = T.GetBranch("e");
    brc->GetEntry(i);
    bre->GetEntry(i);

  If found is not 0, the number of branch(es) found matching the regular
  expression is returned in *found AND the error message 'unknown branch'
  is suppressed.

 -- Set the current branch style.  (static function)

 style = 0 old Branch
 style = 1 new Bronch

 -- Set maximum size of the file cache (default is 10000000, i.e., 10 MB).
 if cachesize <= 0 the existing cache (if any) is deleted
 WARNING: Currently only ONE TTree object can be 'cached' per TFile object.
 This call disable the cache for the other TTree objects read from the same
 TFile object as this TTree (The SetCacheSize called __last__ wins).
 To cache multiple TTree objects in the same ROOT file, you must create
 one TFile object per TTree object.

 -- Enable/Disable circularity for this tree.

 if maxEntries > 0 a maximum of maxEntries is kept in one buffer/basket
 per branch in memory.
   Note that when this function is called (maxEntries>0) the Tree
   must be empty or having only one basket per branch.
 if maxEntries <= 0 the tree circularity is disabled.

 NOTE 1:
  Circular Trees are interesting in online real time environments
  to store the results of the last maxEntries events.
 NOTE 2:
  Calling SetCircular with maxEntries <= 0 is necessary before
  merging circular Trees that have been saved on files.
 NOTE 3:
  SetCircular with maxEntries <= 0 is automatically called
  by TChain::Merge
 NOTE 4:
  A circular Tree can still be saved in a file. When read back,
  it is still a circular Tree and can be filled again.

 -- Set the debug level and the debug range.

 For entries in the debug range, the functions TBranchElement::Fill
 and TBranchElement::GetEntry will print the number of bytes filled
 or read for each branch.

 -- Change the tree's directory.

 Remove reference to this tree from current directory and
 add reference to new directory dir.  The dir parameter can
 be 0 in which case the tree does not belong to any directory.

 -- Change number of entries in the tree.

 If n >= 0, set number of entries in the tree = n.

 If n < 0, set number of entries in the tree to match the
 number of entries in each branch. (default for n is -1)

 This function should be called only when one fills each branch
 independently via TBranch::Fill without calling TTree::Fill.
 Calling TTree::SetEntries() make sense only if the number of entries
 in each branch is identical, a warning is issued otherwise.
 The function returns the number of entries.

Set an EntryList

This function transfroms the given TEventList into a TEntryList
The new TEntryList is owned by the TTree and gets deleted when the tree
is deleted. This TEntryList can be returned by GetEntryList() function, and after
GetEntryList() function is called, the TEntryList is not owned by the tree
any more.

 -- Set number of entries to estimate variable limits.

 -- Set fFileNumber to number.
 fFileNumber is used by TTree::Fill to set the file name
 for a new file to be created when the current file exceeds fgTreeMaxSize.
    (see TTree::ChangeFile)
 if fFileNumber=10, the new file name will have a suffix "_11",
 ie, fFileNumber is incremented before setting the file name

 -- Set the maximum size of a Tree file -- (static function).

 In TTree::Fill, when the file has a size > fgMaxTreeSize,
 the function closes the current file and starts writing into
 a new file with a name of the style "file_1.root" if the original
 requested file name was "file.root".

 -- Change the name of this tree.

 -- Change the name and title of this tree.

 -- The current TreeIndex is replaced by the new index.
 Note that this function does not delete the previous index.
 This gives the possibility to play with more than one index, e.g.,
 TVirtualIndex* oldIndex = tree.GetTreeIndex();
 tree.SetTreeIndex(newIndex);
 tree.Draw();
 tree.SetTreeIndex(oldIndex);
 tree.Draw(); etc

 -- Set tree weight.

 The weight is used by TTree::Draw to automatically weight each
 selected entry in the resulting histogram.

 For example the equivalent of:

      T.Draw("x", "w")

 is:

      T.SetWeight(w);
      T.Draw("x");

 This function is redefined by TChain::SetWeight. In case of a
 TChain, an option "global" may be specified to set the same weight
 for all trees in the TChain instead of the default behaviour
 using the weights of each tree in the chain (see TChain::SetWeight).

 -- Print values of all active leaves for entry.

 if entry==-1, print current entry (default)
 if a leaf is an array, a maximum of lenmax elements is printed.

 -- Start the TTreeViewer on this tree.

  ww is the width of the canvas in pixels
  wh is the height of the canvas in pixels

 -- Stream a class object.

 -- Unbinned fit of one or more variable(s) from a tree.

  funcname is a TF1 function.

  See TTree::Draw for explanations of the other parameters.

   Fit the variable varexp using the function funcname using the
   selection cuts given by selection.

   The list of fit options is given in parameter option.
      option = "Q" Quiet mode (minimum printing)
             = "V" Verbose mode (default is between Q and V)
             = "E" Perform better Errors estimation using Minos technique
             = "M" More. Improve fit results

   You can specify boundary limits for some or all parameters via
        func->SetParLimits(p_number, parmin, parmax);
   if parmin>=parmax, the parameter is fixed
   Note that you are not forced to fix the limits for all parameters.
   For example, if you fit a function with 6 parameters, you can do:
     func->SetParameters(0,3.1,1.e-6,0.1,-8,100);
     func->SetParLimits(4,-10,-4);
     func->SetParLimits(5, 1,1);
   With this setup, parameters 0->3 can vary freely
   Parameter 4 has boundaries [-10,-4] with initial value -8
   Parameter 5 is fixed to 100.

   For the fit to be meaningful, the function must be self-normalized.

   i.e. It must have the same integral regardless of the parameter
   settings.  Otherwise the fit will effectively just maximize the
   area.

   It is mandatory to have a normalization variable
   which is fixed for the fit.  e.g.

     TF1* f1 = new TF1("f1", "gaus(0)/sqrt(2*3.14159)/[2]", 0, 5);
     f1->SetParameters(1, 3.1, 0.01);
     f1->SetParLimits(0, 1, 1); // fix the normalization parameter to 1
     data->UnbinnedFit("f1", "jpsimass", "jpsipt>3.0");


   1, 2 and 3 Dimensional fits are supported.
   See also TTree::Fit

    Return status

   The function return the status of the fit in the following form
     fitResult = migradResult + 10*minosResult + 100*hesseResult + 1000*improveResult
   The fitResult is 0 is the fit is OK.
   The fitResult is negative in case of an error not connected with the fit.
   The number of entries used in the fit can be obtained via
     mytree.GetSelectedRows();
   If the number of selected entries is null the function returns -1

 -- Replace current attributes by current style.

 GetMakeClass is left non-virtual for efficiency reason.
 Making it virtual affects the performance of the I/O