class RooCustomizer: public TNamed, public RooPrintable

 RooCustomizer is a factory class to produce clones
 of a prototype composite PDF object with the same structure but
 different leaf servers (parameters or dependents)

 RooCustomizer supports two kinds of modifications:

 -> replace(leaf_arg,repl_arg)
 replaces each occurence of leaf_arg with repl_arg in the composite pdf.

 -> split(split_arg)
 is used when building multiple clones of the same prototype. Each
 occurrence of split_arg is replaceed with a clone of split_arg
 named split_arg_[MCstate], where [MCstate] is the name of the
 'master category state' that indexes the clones to be built.


 [Example]

 Splitting is particularly useful when building simultaneous fits to
 subsets of the data sample with different background properties.
 In such a case, the user builds a single prototype PDF representing
 the structure of the signal and background and splits the dataset
 into categories with different background properties. Using
 RooCustomizer a PDF for each subfit can be constructed from the
 prototype that has same structure and signal parameters, but
 different instances of the background parameters: e.g.


     RooExponential bg("bg","background",x,alpha) ;
     RooGaussian sig("sig","signal",x,mean,sigma) ;
     RooAddPdf pdf("pdf","pdf",sig,bg,sigfrac) ;

     RooDataSet data("data","dataset",RooArgSet(x,runblock),...)

     RooCategory runblock("runblock","run block") ;
     runblock.defineType("run1") ;
     runblock.defineType("run2") ;

     RooArgSet splitLeafs
     RooCustomizer cust(pdf,runblock,splitLeafs)
     cust.split(alpha,runblock)

     RooAbsPdf* pdf_run1 = cust.build("run1") ;
     RooAbsPdf* pdf_run2 = cust.build("run2") ;

     RooSimultaneous simpdf("simpdf","simpdf",RooArgSet(*pdf_run1,*pdf_run2))

 If the master category state is a super category, leafs may be split
 by any subset of that master category. E.g. if the master category
 is 'A x B', leafs may be split by A, B or AxB.

 In addition to replacing leaf nodes, RooCustomizer clones all branch
 nodes that depend directly or indirectly on modified leaf nodes, so
 that the input pdf is untouched by each build operation.

 The customizer owns all the branch nodes including the returned top
 level node, so the customizer should live as longs as the cloned
 composites are needed.

 Any leaf nodes that are created by the customizer will be put into
 the leaf list that is passed into the customizers constructor (splitLeafs in
 the above example. The list owner is responsible for deleting these leaf
 nodes after the customizer is deleted.


 [Advanced techniques]

 By default the customizer clones the prototype leaf node when splitting a leaf,
 but the user can feed pre-defined split leafs in leaf list. These leafs
 must have the name <split_leaf>_<splitcat_label> to be picked up. The list
 of pre-supplied leafs may be partial, any missing split leafs will be auto
 generated.

 Another common construction is to have two prototype PDFs, each to be customized
 by a separate customizer instance, that share parameters. To ensure that
 the customized clones also share their respective split leafs, i.e.

   PDF1(x,y;A) and PDF2(z,A)   ---> PDF1_run1(x,y,A_run1) and PDF2_run1(x,y,A_run1)
                                    PDF1_run2(x,y,A_run2) and PDF2_run2(x,y,A_run2)

 feed the same split leaf list into both customizers. In that case the second customizer
 will pick up the split leafs instantiated by the first customizer and the link between
 the two PDFs is retained