code: private <T> Collection<Integer> transformUnion(UnionTransformation<T> union) {
  		List<StreamTransformation<T>> inputs = union.getInputs();
  		List<Integer> resultIds = new ArrayList<>();
  
  		for (StreamTransformation<T> input: inputs) {
  			resultIds.addAll(transform(input));
  		}
  
  		return resultIds;
  	}
docs: Transforms a {@code UnionTransformation}.
  
  <p>This is easy, we only have to transform the inputs and return all the IDs in a list so
  that downstream operations can connect to all upstream nodes.