PROPOSAL: Add `disjoint` method to `Data.IntSet`

[Henning Thielemann]

On Tue, 19 Dec 2017, Víctor López Juan wrote:

> I'm thinking that `disjoint` is already a negation:
> (dis- (not) + joint (united)). When composing with `not`, the user gets
> a double negation `not (disjoint x y)`. There is a then a small mental
> effort required to go from "not disjoint" to "overlapping".
>
> If we are going to have only one of the two properties, I would rather
> have the positive one (`overlaps`) as primitive. Then `disjoint` would
> be written "not (overlaps x y)", which reads quite easily.
> (Or even "not (x `overlaps` y)").

I also dislike double negation and think that 'disjoint' is one. I'd 
prefer to see both 'overlap' and 'disjoint'.