intros.
unfold max_nat in |- *.
elim (le_gt_dec n m); auto with arith.
Qed.
Require Export Relation_Definitions.
Definition decide (P : Prop) := {P} + {~ P}.
Hint Unfold decide: core.
Inductive Acc3 (A B C : Set) (R : relation (A * (B * C))) :
A -> B -> C -> Prop :=
Acc3_intro :
forall (x : A) (x0 : B) (x1 : C),
(forall (y : A) (y0 : B) (y1 : C),
R (y, (y0, y1)) (x, (x0, x1)) -> Acc3 R y y0 y1) ->
Acc3 R x x0 x1.

Lemma Acc_Acc3 :
forall (A B C : Set) (R : relation (A * (B * C))) (x : A) (y : B) (z : C),
Acc R (x, (y, z)) -> Acc3 R x y z.