Make_using_map2.1-X
type 'a t
val return : 'a -> 'a t
val map2 : 'a t -> 'b t -> f:( 'a -> 'b -> 'c ) -> 'c t
val map : [ `Define_using_map2 | `Custom of 'a t -> f:( 'a -> 'b ) -> 'b t ]