Rebindable monads?

[Jim Apple]

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#rebindable-syntax

Jim