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

Lemma Forall2_firstn_skipn_iff {A B:Type} (P:A→B→Prop) la lb n: Forall2 P la lb ↔ Forall2 P (firstn n la) (firstn n lb) ∧ Forall2 P (skipn n la) (skipn n lb).