[Haskell-cafe] Patch combinators

Hi all! Based on ideas by Koen Claessen, I have made a small module for what might be called patch combinators: http://hpaste.org/56501 Examples are found as comments. Before I push this to Hackage, I just wanted to check if there is any package that already provides this sort of functionality. (We're planning to use this module for expressing compile-time constraints on Feldspar functions.) Thanks for any feedback! / Emil