We specify that any access to material or tracking media should be done only via a name (character string) instead by a hard-coded number. In general the name of the material and its related tracking media are the same with only one exception. For specific tracking media related to a given subsystem a prefix (a character plus underscore) is added as follows:
T_ -- for the tracker
E_,H_,V_ -- for the electromagnetic, hadron and forward calorimeters,
M_ -- for the muon system and
C_ -- for the general parts.
Furthermore, if the material with this name does not exist, for example
" 
", then the standard material will be used (``Air") and the
tracking model for this media will be set to ``Helix.''
There is one special tracking medium that has the prefix `` 
.''
This tracking medium is used to denote ``thick'' or ``simple'' (i.e.,
without internal structure) shielding materials and iron yokes for the
purpose of reducing computer time.
In the volumes of this kind of medium, kinematical cuts are increased inside
a sub-volume which is reduced from the mother volume by 5 cm in all dimensions.