[Xmonad] darcs patch: add differentiate function to StackSet t... (and 3 more)

This is my robustness set of patches, resent with a conflict resolved. David Tue Jun 12 06:28:53 PDT 2007 David Roundy <droundy@darcs.net> * add differentiate function to StackSet to go [a] -> Stack a. Tue Jun 12 08:05:55 PDT 2007 David Roundy <droundy@darcs.net> * 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 <droundy@darcs.net> * add catchX to catch exceptions. Tue Jun 12 10:06:25 PDT 2007 David Roundy <droundy@darcs.net> * resolve conflict in Operations.