Rebindable monads?

Where is there documentation for rebindable syntax for monads with class constraints: (>>=) :: (Foo m, Baz a) => m a -> (a -> m b) -> m b http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/9447/match=syntax The users guide seems to disallow such type signatures: http://haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#rebind... Jim