fact: Fixpoint fold_left2 (xs ys: list B) (acc:A) {struct xs} : A := match xs, ys with | nil, _ | _, nil => acc | x::xs, y::ys => fold_left2 xs ys (f acc x y) end.
imports: List Bool NArith Psatz Int63 Nnat ZArith Misc ProofIrrelevance