fact: Fixpoint map2 (l1 : list A) : list B -> list C := fun l2 => match l1 with | [] => [] | a :: l3 => match l2 with | [] => [] | b :: l4 => f a b :: map2 l3 l4 end end.
imports: Coq(Inverse_Image Wf_nat Sorting.Permutation)