[Xmonad] darcs patch: add differentiate function to StackSet t... (and 2 more)

Here's the code to catch exceptions. I currently catch exceptions when calling layouts. We could add catchX calls elsewhere, if it seems advisable, but I think this is sufficient. 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. Tue Jun 12 08:42:53 PDT 2007 David Roundy * add catchX to catch exceptions.