(** Same as [map], but the function does not receive the [elt] argument. *)
Parameter map1:
forall (A B: Type), (A -> B) -> t A -> t B.

Axiom gmap1:
forall (A B: Type) (f: A -> B) (i: elt) (m: t A),
get i (map1 f m) = option_map f (get i m).