code: public ExecutionVertex getInputVertex(final int stage, final int index) {
  
  		try {
  			final ExecutionStage s = this.stages.get(stage);
  			if (s == null) {
  				return null;
  			}
  
  			return s.getInputExecutionVertex(index);
  
  		} catch (ArrayIndexOutOfBoundsException e) {
  			return null;
  		}
  	}
docs: Returns the input vertex with the specified index for the given stage
  
  @param stage
  the index of the stage
  @param index
  the index of the input vertex to return
  @return the input vertex with the specified index or <code>null</code> if no input vertex with such an index
  exists in that stage