class TGeoVolumeAssembly: public TGeoVolume

   TGeoVolume - the base class representing solids.

   Volumes are the basic objects used in building the geometrical hierarchy.
 They represent unpositioned objects but store all information about the
 placement of the other volumes they may contain. Therefore a volume can
 be replicated several times in the geometry. In order to create a volume, one
 has to put togeather a shape and a medium which are already defined. Volumes
 have to be named by users at creation time. Every different name may represent a
 an unique volume object, but may also represent more general a family (class)
 of volume objects having the same shape type and medium, but possibly
 different shape parameters. It is the user's task to provide different names
 for different volume families in order to avoid ambiguities at tracking time.
 A generic family rather than a single volume is created only in two cases :
 when a generic shape is provided to the volume constructor or when a division
 operation is applied. Each volume in the geometry stores an unique
 ID corresponding to its family. In order to ease-up their creation, the manager
 class is providing an API that allows making a shape and a volume in a single step.

   Volumes are objects that can be visualized, therefore having visibility,
 colour, line and fill attributes that can be defined or modified any time after
 the volume creation. It is advisable however to define these properties just
 after the first creation of a volume namespace, since in case of volume families
 any new member created by the modeler inherits these properties.

    In order to provide navigation features, volumes have to be able to find
 the proper container of any point defined in the local reference frame. This
 can be the volume itself, one of its positioned daughter volumes or none if
 the point is actually outside. On the other hand, volumes have to provide also
 other navigation methods such as finding the distances to its shape boundaries
 or which daughter will be crossed first. The implementation of these features
 is done at shape level, but the local mother-daughters management is handled
 by volumes that builds additional optimisation structures upon geometry closure.
 In order to have navigation features properly working one has to follow the
 general rules for building a valid geometry (see TGeoManager class).

   Now let's make a simple volume representing a copper wire. We suppose that
 a medium is already created (see TGeoMedium class on how to create media).
 We will create a TUBE shape for our wire, having Rmin=0cm, Rmax=0.01cm
 and a half-length dZ=1cm :

   TGeoTube *tube = new TGeoTube("wire_tube", 0, 0.01, 1);

 One may ommit the name for the shape if no retreiving by name is further needed
 during geometry building. The same shape can be shared by different volumes
 having different names and materials. Now let's make the volume for our wire.
 The prototype for volumes constructor looks like :

   TGeoVolume::TGeoVolume(const char *name, TGeoShape *shape, TGeoMedium *med)

 Since TGeoTube derives brom the base shape class, we can provide it to the volume
 constructor :

   TGeoVolume *wire_co = new TGeoVolume("WIRE_CO", tube, ptrCOPPER);

 Do not bother to delete neither the media, shapes or volumes that you have
 created since all will be automatically cleaned on exit by the manager class.
 If we would have taken a look inside TGeoManager::MakeTube() method, we would
 have been able to create our wire with a single line :

   TGeoVolume *wire_co = gGeoManager->MakeTube("WIRE_CO", ptrCOPPER, 0, 0.01, 1);

 The same applies for all primitive shapes, for which there can be found
 corresponding MakeSHAPE() methods. Their usage is much more convenient unless
 a shape has to be shared between more volumes. Let's make now an aluminium wire
 having the same shape, supposing that we have created the copper wire with the
 line above :

   TGeoVolume *wire_al = new TGeoVolume("WIRE_AL", wire_co->GetShape(), ptrAL);

 Now that we have learned how to create elementary volumes, let's see how we
 can create a geometrical hierarchy.


   Positioning volumes


   When creating a volume one does not specify if this will contain or not other
 volumes. Adding daughters to a volume implies creating those and adding them
 one by one to the list of daughters. Since the volume has to know the position
 of all its daughters, we will have to supply at the same time a geometrical
 transformation with respect to its local reference frame for each of them.
 The objects referencing a volume and a transformation are called NODES and
 their creation is fully handled by the modeler. They represent the link
 elements in the hierarchy of volumes. Nodes are unique and distinct geometrical
 objects ONLY from their container point of view. Since volumes can be replicated
 in the geometry, the same node may be found on different branches.

   An important observation is that volume objects are owned by the TGeoManager
 class. This stores a list of all volumes in the geometry, that is cleaned
 upon destruction.

   Let's consider positioning now our wire in the middle of a gas chamber. We
 need first to define the gas chamber :

   TGeoVolume *chamber = gGeoManager->MakeTube("CHAMBER", ptrGAS, 0, 1, 1);

 Now we can put the wire inside :

   chamber->AddNode(wire_co, 1);

 If we inspect now the chamber volume in a browser, we will notice that it has
 one daughter. Of course the gas has some container also, but let's keep it like
 that for the sake of simplicity. The full prototype of AddNode() is :

   TGeoVolume::AddNode(TGeoVolume *daughter, Int_t usernumber,
                       TGeoMatrix *matrix=gGeoIdentity)

 Since we did not supplied the third argument, the wire will be positioned with
 an identity transformation inside the chamber. One will notice that the inner
 radii of the wire and chamber are both zero - therefore, aren't the two volumes
 overlapping ? The answer is no, the modeler is even relaying on the fact that
 any daughter is fully contained by its mother. On the other hand, neither of
 the nodes positioned inside a volume should overlap with each other. We will
 see that there are allowed some exceptions to those rules.

 Overlapping volumes


   Positioning volumes that does not overlap their neighbours nor extrude
 their container is sometimes quite strong contrain. Some parts of the geometry
 might overlap naturally, e.g. two crossing tubes. The modeller supports such
 cases only if the overlapping nodes are declared by the user. In order to do
 that, one should use TGeoVolume::AddNodeOverlap() instead of TGeoVolume::AddNode().
   When 2 or more positioned volumes are overlapping, not all of them have to
 be declared so, but at least one. A point inside an overlapping region equally
 belongs to all overlapping nodes, but the way these are defined can enforce
 the modeler to give priorities.
   The general rule is that the deepest node in the hierarchy containing a point
 have the highest priority. For the same geometry level, non-overlapping is
 prioritized over overlapping. In order to illustrate this, we will consider
 few examples. We will designate non-overlapping nodes as ONLY and the others
 MANY as in GEANT3, where this concept was introduced:
   1. The part of a MANY node B extruding its container A will never be "seen"
 during navigation, as if B was in fact the result of the intersection of A and B.
   2. If we have two nodes A (ONLY) and B (MANY) inside the same container, all
 points in the overlapping region of A and B will be designated as belonging to A.
   3. If A an B in the above case were both MANY, points in the overlapping
 part will be designated to the one defined first. Both nodes must have the
 same medium.
   4. The silces of a divided MANY will be as well MANY.

 One needs to know that navigation inside geometry parts MANY nodes is much
 slower. Any overlapping part can be defined based on composite shapes - this
 is always recommended.