
               **************
               ***  DEMO  ***
               **************

 --- MAIN : Visualize the detector, using OpenGL or VRML,
     to see if it looks reasonable. Some very simple kind
     of user interface must also be added.

     Move the "hardwire" geant4 commands from the main to
     macro file: macro.g4.
     By running:   myMain macro.g4
     you should obtain the same results in the previous step.
     If instead you run:  myMain
     then a display of the detector should appear, together
     with the Geant4 command prompt  Idle> .
     An useful visualization command is, for example:
              Idle> /vis/viewer/viewpointThetaPhi 30 45

 --- MyVisManager : create this class, which inherits from 
     G4VisManager, and provides a simple implementation of the 
     Visualization Manager.

 --- DETECTOR : in the  Construct()  method the visualization
     attributes (visibility, colour, etc.) for each logical
     volume should be set. Select: world invisible; absorber
     white; active yellow.
 