Definition ints (n m:nat):list nat := seq n (m-n).

Lemma Forall2_app_inv {A B:Type} (P:A→B→Prop) la1 la2 lb1 lb2: length la1 = length lb1 → Forall2 P (la1++la2) (lb1++lb2) → Forall2 P la1 lb1 ∧ Forall2 P la2 lb2.