code: public void addFeedbackEdge(StreamTransformation<T> transform) {
  
  		if (transform.getParallelism() != this.getParallelism()) {
  			throw new UnsupportedOperationException(
  					"Parallelism of the feedback stream must match the parallelism of the original" +
  							" stream. Parallelism of original stream: " + this.getParallelism() +
  							"; parallelism of feedback stream: " + transform.getParallelism() +
  							". Parallelism can be modified using DataStream#setParallelism() method");
  		}
  
  		feedbackEdges.add(transform);
  	}
docs: Adds a feedback edge. The parallelism of the {@code StreamTransformation} must match
  the parallelism of the input {@code StreamTransformation} of this
  {@code FeedbackTransformation}
  
  @param transform The new feedback {@code StreamTransformation}.