How are there more cases to test with less arguments? For a given case, when there are two streams, I need to test that case for the right and the left stream. If there are n streams, I need to test that case for 0 streams, 1 stream, and then for n streams (testing whether the case applies to the first stream, a middle stream, or the last stream). Even if I've done that I haven't exhaustively tested all the cases (since there are an infinite number) - I am relying on induction to show that it holds for any n.