Make3.1-X
type ('a, 'd, 'e) t
val return : 'a -> ('a, _, _) t
val apply : ('a -> 'b, 'd, 'e) t -> ('a, 'd, 'e) t -> ('b, 'd, 'e) t
val map : [ `Define_using_apply | `Custom of ('a, 'd, 'e) t -> f:('a -> 'b) -> ('b, 'd, 'e) t ]