code: public int getNumberOfInputGroupVertices() {
  
  		int retVal = 0;
  
  		final Iterator<ManagementGroupVertex> it = this.groupVertices.iterator();
  		while (it.hasNext()) {
  			if (it.next().isInputVertex()) {
  				++retVal;
  			}
  		}
  
  		return retVal;
  	}
docs: Returns the number of input group vertices in this stage. Input group vertices are those vertices which have
  incoming edges from group vertices of a lower stage.
  
  @return the number of input group vertices in this stage