Set difference is symmetrical difference.

[AljoschaMeyer]

XyzSet::difference computes the symmetrical difference, not regular difference (aka relative complement). That was fairly surprising to me, it might be a good idea to document this more explicitly than only in the example.

If this was a deliberate choice of terminology, I'd be interested in hearing about the reasoning. Also, would you be open to adding XyzSet::relative_complement to the crate?

[bodil]

It was deliberate only in the sense that I've recycled the method names from equivalent data structure impls in Haskell, so I couldn't tell you the actual reasoning behind them.

I'd be very happy to take PRs for any additional methods that make sense, and I don't see why this wouldn't. 😊