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               ***  DEMO  ***
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 Consider the active gas layer, of the first part of the 
 detector, as the only sensitive part. 
 Define a "calorimeter-like" hit type for such sensitive layer,
 that is, create one hit (for event) only the first time
 some energy is deposited in such layer: then, add to it
 any other energy deposit which happens in the same event.
 To do it, you need three things:

 --- HIT CLASS : write an Hit class.

 --- SENSITIVE DETECTOR : write a Sensitive Detector class which,
     in the  ProcessHits  method, if there is a non-null
     energy deposit, creates the above Hit in the case this has not 
     been already created, otherwise it simply adds (cumulates) the 
     energy deposit.

 --- DETECTOR : the above sensitive detector must be associated
     to the active gas layer of the first part of the detector.
 
 
 NB) Physically, you could imagine the above situation
     as a typical single calorimeter tower in a test beam.
