- Tasks blocking xmonad 0.1
    * CRITICAL: fix killClient code. see for example, xclock.
    - more quickchecks (layout algorithm is one candidate)
    - website, xmonad.org
    - testing



- let mod+enter demote a master window

- think about the statusbar/multithreading.
    Three shared TVars:
        windowTitle :: TVar String
        workspace   :: TVar Int
        statusText  :: TVar String
    Three threads:
        Main thread, handles all of the events that it handles now.  When
        necessary, it writes to workspace or windowTitle

        Status IO thread, the algorithm is something like this:
            forever $ do
                s <- getLine
                atomic (writeTVar statusText s)

        Statusbar drawing thread, waits for changes in all three TVars, and
        redraws whenever it finds a change.

- Notes on new StackSet:

   The actors: screens, workspaces, windows

   Invariants:
   - There is exactly one screen in focus at any given time.
   - A screen views exactly one workspace.
   - A workspace is visible on one or zero screens.
   - A workspace has zero or more windows.
   - A workspace has either one or zero windows in focus.  Zero if the
     workspace has no windows, one in all other cases.
   - A window is a member of only one workspace.