Re: [Haskell-cafe] Re: what is inverse of mzero and return?

Just thinking about this, a monad is a Functor plus two natural-tranformations, Unit and Join. Is there an equivalent definition for MonadPlus... I am not sure I understand where MonadPlus comes from? Is it just a Functor and two different definitions of Unit and Join (from those chosen to be in the class Monad?) Keean.