Pair2
val pair : 'a -> 'b -> 'a * 'b
val map_fst : ('a -> 'c) -> ('a * 'b) -> 'c * 'b
val map_snd : ('b -> 'c) -> ('a * 'b) -> 'a * 'c
val lift_snd_result : ('a * ('b, 'e) Stdlib.result) -> ('a * 'b, 'e) Stdlib.result