Section Motivation.
Variable (n: nat).
Implicit Type (m p: nat) (M N: list (nat * nat)).
Definition step '(a, b) := (n + a, 1 + b).
Notation Step X := (map step X).
Definition t k := (k * n, k).
Definition T k := tab t k.
Definition Mrel m p M := M ++ [(p, m)] = t 0 :: Step M.
enough (forall a b x, M ++ [x] = (a, b) :: Step M -> M ++ [x] = tab (fun k => (a + k * n, b + k)) (S (length M))) as H.
- intros H1 % H; apply app_injective_right in H1 as [H1 _]; eauto.
- induction M as [|[l r]]; [cbn|]; intros; simplify; eauto.
injection H as ? ? H1; subst. specialize (IHM _ _ _ H1) as ->.
cbn [length]; rewrite !tab_S; cbn; simplify; do 2 f_equal.
f_equal; lia. eapply tab_ext; intros; f_equal; lia.
Qed.

Lemma M_backward_equations m p l : Mrel m p (T l) -> m = l /\ m * n = p.