Definition R_noet (x y : nat * nat) : Prop :=
  ((fst x) + (snd x) < (fst y) + (snd y))%nat.Definition f (x : nat * nat) := ((fst x) + (snd x))%nat.

Lemma infinite_descent : forall P : Z -> Z -> Prop, (forall x1 x2 : Z, 0 <= x1 -> 0 <= x2 -> (P x1 x2 -> exists y1 : Z, exists y2 : Z, 0 <= y1 /\ 0 <= y2 /\ y1 + y2 < x1 + x2 /\ P y1 y2)) -> forall x y: Z, 0 <= x -> 0 <= y -> ~(P x y).