$Id: BUG.userStackingAction,v 1.1 2002/03/05 15:22:07 gcosmo Exp $
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When Geant4 4.0 sees empty event, it still calls userStackingAction.

In the function:
  G4StackManager::PopNextTrack(G4VTrajectory**newTrajectory)

in Geant4 4.0, one has:

145     if(userStackingAction) userStackingAction->NewStage();
146 #ifdef G4VERBOSE
147     if( verboseLevel > 0 ) G4cout << "     " << GetNUrgentTrack()
148                       << " urgent tracks and " << GetNWaitingTrack()
149                       << " waiting tracks." << G4endl;
150 #endif
151     if( ( GetNUrgentTrack()==0 ) && ( GetNWaitingTrack()==0 ) ) return 0;

It seems to me that the call, `return 0' (line 151), should occur before
the call to `NewStage()' (line 145).  If there are no tracks to
be stacked (neither GetNUrgentTrack() nor GetNWaitingTrack()),
then there is no new stage.  So, it's a mistake to call
  userStackingAction->NewStage().

This affects my parallelization of Geant4, in which I need to purposely
create a trivial event on the master, and call
G4EventManager()::ProcessOneEvent() on it to initialize the event on the
master.  I am doing this in order to copy the hits from the slave into
an event on the master.