Inductive Forall2' (A B : Type) (R : A -> B -> Type) : list A -> list B -> Type :=
Forall2'_nil : Forall2' R [] []
| Forall2'_cons : forall (x : A) (y : B) (l : list A) (l' : list B),
R x y -> Forall2' R l l' -> Forall2' R (x :: l) (y :: l').

Lemma Forall2_product (A : Type) (R : A -> A -> Prop) (ls lt : list A) :
ls <>  [] -> Forall2 R ls lt -> product R ls lt.