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.

Theorem gmap: forall (A B: Type) (f: positive -> A -> B) (i: positive) (m: t A), get i (map f m) = option_map (f i) (get i m).