I want to preface this by saying that I think that diffing methods would be a great addition to the standard library, I only disagree with mingling this with the ordered collection issue.
Depends what you mean by “on Collection” but there are several methods inherited from Sequence that are very difficult to think of a reasonable use for.
This seems like the wrong order to me. It hasn't been decided that OrderedCollection is the right way to address this issue, so making this proposal contingent on OrderedCollection entangles the two issues unnecessarily (and, in my opinion, counterproductively).
Sure, I don't want to push for a particular solution here, or really discuss it at all, I'm only arguing that this proposal is the wrong level to make this decision at because it has much broader implications for a controversial topic. I would really prefer either that the “ordered” collection question is discussed/solved before it's decided where these diffing methods should live, or that this proposal ignores that orthogonal issue entirely and leaves it to be dealt with holistically later.