[Xmonad] darcs patch: add differentiate function to StackSet t... (and 1 more)

[Donald Bruce Stewart]

droundy:
> Hi all,
> 
> This is my change to make (Stack a) never be empty, with possibly empty
> stacks represented by (Maybe Stack a).  This slightly simplifies the layout
> code, since we handle the case of no windows in the core.  This assumes
> that we don't have a layout that needs to do something with an empty
> screen.

This looks ok. Some code gets easier, some gets harder, but we do get a
type for non-empty workspaces. So that's good.

> Anyhow, one step in my campaign for powerful, safe extensibility.  The main
> thing we need to do, though, is to catch exceptions...

Great!

-- Don