Fixpoint xelements (A : Type) (m : t A) (i : positive)
                       (k: list (positive * A)) {struct m}
                       : list (positive * A) :=
      match m with
      | Leaf => k
      | Node l None r =>
          xelements l (xO i) (xelements r (xI i) k)
      | Node l (Some x) r =>
          xelements l (xO i)
            ((prev i, x) :: xelements r (xI i) k)
      end.Definition elements (A: Type) (m : t A) := xelements m xH nil.Remark xelements_append:
    forall A (m: t A) i k1 k2,
    xelements m i (k1 ++ k2) = xelements m i k1 ++ k2.Remark xelements_leaf:
    forall A i, xelements (@Leaf A) i nil = nil.Remark xelements_node:
    forall A (m1: t A) o (m2: t A) i,
    xelements (Node m1 o m2) i nil =
       xelements m1 (xO i) nil
    ++ match o with None => nil | Some v => (prev i, v) :: nil end
    ++ xelements m2 (xI i) nil.

Lemma xelements_incl: forall (A: Type) (m: t A) (i : positive) k x, In x k -> In x (xelements m i k).