Section COMBINE.
Variables A B C: Type.
Variable f: option A -> option B -> option C.
Hypothesis f_none_none: f None None = None.
Fixpoint xcombine_l (m : t A) {struct m} : t C :=
match m with
| Leaf => Leaf
| Node l o r => Node' (xcombine_l l) (f o None) (xcombine_l r)
end.
Fixpoint xcombine_r (m : t B) {struct m} : t C :=
match m with
| Leaf => Leaf
| Node l o r => Node' (xcombine_r l) (f None o) (xcombine_r r)
end.
Fixpoint combine (m1: t A) (m2: t B) {struct m1} : t C :=
match m1 with
| Leaf => xcombine_r m2
| Node l1 o1 r1 =>
match m2 with
| Leaf => xcombine_l m1
| Node l2 o2 r2 => Node' (combine l1 l2) (f o1 o2) (combine r1 r2)
end
end.
End COMBINE.
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 in_xelements:
forall (A: Type) (m: t A) (i k: positive) (v: A) ,
In (k, v) (xelements m i nil) ->
exists j, k = prev (prev_append j i) /\ get j m = Some v.