That was Ashley's original proposal, but SimonM responded in

http://www.haskell.org/pipermail/glasgow-haskell-users/2001-September/00 2290.html

http://www.haskell.org/pipermail/glasgow-haskell-users/2001-September/00 2304.html I think I had the wrong end of the stick; or at least I hadn't appreciated the extreme cleverness of the idea. I think I've got it now, though ;-) Cheers, Simon