[Xmonad] darcs patch: Introduce NonEmptyStack.

[Donald Bruce Stewart]

droundy:
> Hi all,
> 
> Here's a patch implementing a NonEmptyStack type that is our
> zipper without the Empty possibility.  I like this change, but
> haven't cleaned up the comments as it's not clear that others
> will like it.  I made modify accept a function that will only
> modify NonEmptyStacks, which covers all the possibilities but
> delete.


i'm open to this. Just want to think about it a bit. Another option
I used was to change the type of with's second argument, to take a
non-empty stack type. 

I'll look at this.

-- Don