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)).
The signature is exactly the same as for S1 except S.unit which got
pruned. (We cannot write let unit = return () without forcing the
'd parameter of ('a, 'd) monad to be covariant.)
type ('a, 'b) t
val bind : ('a, 'b) t ->
('a -> ('c, 'b) t) -> ('c, 'b) tval return : 'a -> ('a, 'b) t