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;
	}
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 ; }
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.