class TGeoPhysicalNode: public TNamed, public TAttLine

 TGeoPhysicalNode, TGeoPNEntry

 Physical nodes are the actual 'touchable' objects in the geometry, representing
 a path of positioned volumes starting with the top node:
    path=/TOP/A_1/B_4/C_3 , where A, B, C represent names of volumes.
 The number of physical nodes is given by the total number of possible of
 branches in the geometry hierarchy. In case of detector geometries and
 specially for calorimeters this number can be of the order 1e6-1e9, therefore
 it is impossible to create all physical nodes as objects in memory. In TGeo,
 physical nodes are represented by the class TGeoPhysicalNode and can be created
 on demand for alignment purposes:

    TGeoPhysicalNode *pn = new TGeoPhysicalNode("path_to_object")

 Once created, a physical node can be misaligned, meaning that its position
 or even shape can be changed:

    pn->Align(TGeoMatrix* newmat, TGeoShape* newshape, Bool_t check=kFALSE)

 The knowledge of the path to the objects that need to be misaligned is
 essential since there is no other way of identifying them. One can however
 create 'symbolic links' to any complex path to make it more representable
 for the object it designates:

    TGeoPNEntry *pne = new TGeoPNEntry("TPC_SECTOR_2", "path_to_tpc_sect2");
    pne->SetPhysicalNode(pn)

 Such a symbolic link hides the complexity of the path to the align object and
 replaces it with a more meaningful name. In addition, TGeoPNEntry objects are
 faster to search by name and they may optionally store an additional user
 matrix.

 For more details please read the misalignment section in the Users Guide.