Goal:
    A : Type  B : Type  P : list A → list A → A → A → B → Type
  ============================
   (([] = []) +
    All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) []) [] [])%type


DEBUG:Going to execute:
match goal with
           | H:All2 _ ?x ?y
             |- _ =>
                 (first [ is_ctor_list x | is_ctor_list y ]); inversion H;
                  clear H
           | H:All2_fold _ ?x ?y
             |- _ =>
                 (first [ is_ctor_list x | is_ctor_list y ]); inversion H;
                  clear H
           | H:All _ ?x |- _ => is_ctor_list x; inversion H; clear H
           | |- (_ :: _ = []) + _ => right
           | |- (?x = ?x) + _ => left
           end