Multi parameter monad.
The second parameter get unified across all the computation. This is used
to encode monads working on a multi parameter data structure like
(('a,'b result)).
type('a, 'd) t
val bind : ('a, 'd) t -> ('a -> ('b, 'd) t) -> ('b, 'd) t
val map : ('a, 'd) t -> f:('a -> 'b) -> ('b, 'd) t