Remark xelements_empty:
    forall (A: Type) (m: t A) i, (forall i, get i m = None) -> xelements m i nil = nil.

(* Specifications: *)
Theorem elements_canonical_order': forall (A B: Type) (R: A -> B -> Prop) (m: t A) (n: t B), (forall i, option_rel R (get i m) (get i n)) -> list_forall2 (fun i_x i_y => fst i_x = fst i_y /\ R (snd i_x) (snd i_y)) (elements m) (elements n).
Theorem elements_canonical_order: forall (A B: Type) (R: A -> B -> Prop) (m: t A) (n: t B), (forall i x, get i m = Some x -> exists y, get i n = Some y /\ R x y) -> (forall i y, get i n = Some y -> exists x, get i m = Some x /\ R x y) -> list_forall2 (fun i_x i_y => fst i_x = fst i_y /\ R (snd i_x) (snd i_y)) (elements m) (elements n).
Theorem elements_extensional: forall (A: Type) (m n: t A), (forall i, get i m = get i n) -> elements m = elements n.
Lemma xelements_remove: forall (A: Type) v (m: t A) i j, get i m = Some v -> exists l1 l2, xelements m j nil = l1 ++ (prev (prev_append i j), v) :: l2 /\ xelements (remove i m) j nil = l1 ++ l2.
Theorem elements_remove: forall (A: Type) i v (m: t A), get i m = Some v -> exists l1 l2, elements m = l1 ++ (i,v) :: l2 /\ elements (remove i m) = l1 ++ l2.