module type Basic2 =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 (sig
..end
('a,'b result)
).type ('a, 'd)
t
val bind : ('a, 'd) t ->
('a -> ('b, 'd) t) -> ('b, 'd) t
val return : 'a -> ('a, 'b) t