code: private <T> Collection<Integer> transformSplit(SplitTransformation<T> split) {
  
  		StreamTransformation<T> input = split.getInput();
  		Collection<Integer> resultIds = transform(input);
  
  		validateSplitTransformation(input);
  
  		// the recursive transform call might have transformed this already
  		if (alreadyTransformed.containsKey(split)) {
  			return alreadyTransformed.get(split);
  		}
  
  		for (int inputId : resultIds) {
  			streamGraph.addOutputSelector(inputId, split.getOutputSelector());
  		}
  
  		return resultIds;
  	}
docs: Transforms a {@code SplitTransformation}.
  
  <p>We add the output selector to previously transformed nodes.