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

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. Anyhow, one step in my campaign for powerful, safe extensibility. The main thing we need to do, though, is to catch exceptions... David Tue Jun 12 06:28:53 PDT 2007 David Roundy * add differentiate function to StackSet to go [a] -> Stack a. Tue Jun 12 08:05:55 PDT 2007 David Roundy * make focus, up and down complete functions. This is a rerun of my change to make (Stack a) never be empty. Gives us more type-safety.