Multi parameter monad. The second and third parameters get unified across all the computation.
type ('a, 'd, 'e) t
val bind : ('a, 'd, 'e) t ‑> f:('a ‑> ('b, 'd, 'e) t) ‑> ('b, 'd, 'e) t
val map : [ `Define_using_bind | `Custom of ('a, 'd, 'e) t ‑> f:('a ‑> 'b) ‑> ('b, 'd, 'e) t ]
val return : 'a ‑> ('a, _, _) t