End COMBINE.

(* Specifications: *)
Lemma xcombine_lr :
forall (A B: Type) (f g : option A -> option A -> option B) (m : t A),
(forall (i j : option A), f i j = g j i) ->
xcombine_l f m = xcombine_r g m.
Theorem combine_commut:
forall (A B: Type) (f g: option A -> option A -> option B),
(forall (i j: option A), f i j = g j i) ->
forall (m1 m2: t A),
combine f m1 m2 = combine g m2 m1.