Fixpoint map1 (A B: Type) (f: A -> B) (m: t A) {struct m} : t B :=
match m with
| Leaf => Leaf
| Node l o r => Node (map1 f l) (option_map f o) (map1 f r)
end.

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