docs: 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;
  	}