code: public ExecutionVertex getOutputExecutionVertex(int index) {
  
  		final Iterator<ExecutionGroupVertex> it = this.stageMembers.iterator();
  		while (it.hasNext()) {
  
  			final ExecutionGroupVertex groupVertex = it.next();
  			if (groupVertex.isOutputVertex()) {
  				final int numberOfMembers = groupVertex.getCurrentNumberOfGroupMembers();
  				if (index >= numberOfMembers) {
  					index -= numberOfMembers;
  				} else {
  					return groupVertex.getGroupMember(index);
  				}
  			}
  		}
  
  		return null;
  	}
docs: Returns the input execution vertex with the given index or <code>null</code> if no such vertex exists.
  
  @param index
  the index of the vertex to be selected.
  @return the input execution vertex with the given index or <code>null</code> if no such vertex exists