Hello programmers,

I have a problem to solve and don't have a clue where to start.

This is a homework question. Please read http://www.haskell.org/hawiki/HomeworkHelp for some helpful hints. By the way, you shouldn't post the same message to multiple lists - you should choose the right one (haskell-cafe in this case), or if you must, cross-post one message to both. This means people who subscribe to both won't see your message twice. --KW 8-) -- Keith Wansbrough http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.