XPrompt: quickcheck related refactoring
XPrompt: quickcheck related refactoring

With this patch XPrompt can be tested with quickcheck. As a result
getLastWord and skipLastWord has been refactored to avoid possible
exceptions.

darcs-hash:20070817155454-32816-cf913d1207efe2c090c150bf192ba737323c4593

yzhou61/xmonad-contrib,yzhou61/xmonad-contrib,yzhou61/xmonad-contrib