darcs patch: add Data.Set.notMember and Data.Map.notMember

[Neil Mitchell]

Since everyone is disagreeing, and I think these are a good idea, I
thought I'd just say so.

> only a single obvious interpretation of meaning so people likely to
> define their own with same name conflicting with each other and it
> won't hurt to provide them.
This is the killer reason

If "not" is so easy and simple, why do we have both break and span ;)

Thanks

Neil