End Append.
Section Forall2.

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).