Fixpoint xmap (A B : Type) (f : positive -> A -> B) (m : t A) (i : positive)
{struct m} : t B :=
match m with
| Leaf => Leaf
| Node l o r => Node (xmap f l (xO i))
(match o with None => None | Some x => Some (f (prev i) x) end)
(xmap f r (xI i))
end.
Definition map (A B : Type) (f : positive -> A -> B) m := xmap f m xH.

Lemma xgmap:
forall (A B: Type) (f: positive -> A -> B) (i j : positive) (m: t A),
get i (xmap f m j) = option_map (f (prev (prev_append i j))) (get i m).