code: public ManagementGroupVertex getInputGroupVertex(final int stage, final int index) {
  
  		if (stage >= this.stages.size()) {
  			return null;
  		}
  
  		return this.stages.get(stage).getInputGroupVertex(index);
  	}
docs: Returns the input group vertex at the given index in the given stage.
  
  @param stage
  the index to the management stage
  @param index
  the index to the input group vertex
  @return the input group vertex at the given index in the given stage or <code>null</code> if either the stage
  does not exists or the given index is invalid in this stage