Thank you for the comments! New version of the patch - per earlier suggestions added examples for corner cases. The patch is attached to the ticket on http://hackage.haskell.org/trac/ghc/ticket/1034 Here I attached HTML file showing Haddock results for the affected part of documentation. Hope this is easier to read than patch. I decided to leave QuickCheck-style properties out of scope for this patch. Andriy ____________________________________________________________________________________ Do you Yahoo!? Everyone is raving about the all-new Yahoo! Mail beta. http://new.mail.yahoo.com