code: public boolean isInputVertex() {
  
  		if (this.backwardEdges.size() == 0) {
  			return true;
  		}
  
  		final Iterator<ManagementGroupEdge> it = this.backwardEdges.iterator();
  		while (it.hasNext()) {
  			if (it.next().getSource().getStageNumber() == this.getStageNumber()) {
  				return false;
  			}
  		}
  
  		return true;
  	}
docs: Checks if this vertex is an input vertex in its stage, i.e. has either no
  incoming connections or only incoming connections to group vertices in a lower stage.
  
  @return <code>true</code> if this vertex is an input vertex, <code>false</code> otherwise