Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  c : C
  a : C
  ls1 : list C
  IH : ∀ (ls2 : list A) (ls3 : list B),
         All (λ z : C, All2 (λ (x : A) (y : B), P x y z) ls2 ls3) (c :: ls1)
         → All2 (λ (x : A) (y : B), All (P x y) (c :: ls1)) ls2 ls3
  a0 : A
  ls2 : list A
  b : B
  ls3 : list B
  X : P a0 b c
  X2 : All2 (λ (x : A) (y : B), P x y c) ls2 ls3
  X3 : All
         (λ z : C, All2 (λ (x : A) (y : B), P x y z) (a0 :: ls2) (b :: ls3))
         ls1
  X1 : P a0 b a
  X4 : All2 (λ (x : A) (y : B), P x y a) ls2 ls3
  ============================
   (All (P a0 b) ls1)


DEBUG:Going to execute:
(first
            [ eapply All2_impl | eapply All2_fold_impl | eapply All_impl ]);
            tea; cbn; intros *; inversion 1 + constructor; subst; eauto
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.