In this case, due to the rules, it would be possible; the call to clone would be inside the method call, so you wouldn't see it. (With other signatures it may not, but Add returns a value, not a reference.)

In other words, it would violate the conventions, but not the rules.

(Okay so there's a _really_ obscure rule here that sill prevents this but it only matters because it's the standard library. It's not actually the signature)