Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  a : C
  a0 : A
  ls2 : list A
  b : B
  ls3 : list B
  X : P a0 b a
  X2 : All2 (λ (x : A) (y : B), P x y a) ls2 ls3
  ============================
   (All2 (λ (x : A) (y : B), All (P x y) []) ls2 ls3)


DEBUG:Going to execute:
(first
            [ eapply All2_impl | eapply All2_fold_impl | eapply All_impl ]);
            tea; cbn; intros *; inversion 1 + constructor; subst