Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  a : B
  ls1 : list B
  a0 : A
  ls2 : list A
  a1 : A
  ls3 : list A
  X0 : All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) (a :: ls1))
         ls2 ls3
  X : P ls2 ls3 a0 a1 a
  X2 : All (P ls2 ls3 a0 a1) ls1
  ============================
   (All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) ls1) ls2 ls3)


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

In environment
A : Type
B : Type
P : list A → list A → A → A → B → Type
a : B
ls1 : list B
a0 : A
ls2 : list A
a1 : A
ls3 : list A
X0 : All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) (a :: ls1))
       ls2 ls3
X : P ls2 ls3 a0 a1 a
X2 : All (P ls2 ls3 a0 a1) ls1
Unable to unify "All2_fold ?M3411 (?M3412 :: ?M3414) (?M3413 :: ?M3415)" with
 "All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) ls1) ls2 ls3".
Level 0: Backtrace:

In environment
A : Type
B : Type
P : list A → list A → A → A → B → Type
a : B
ls1 : list B
a0 : A
ls2 : list A
a1 : A
ls3 : list A
X0 : All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) (a :: ls1))
       ls2 ls3
X : P ls2 ls3 a0 a1 a
X2 : All (P ls2 ls3 a0 a1) ls1
Unable to unify "All2_fold ?M3411 (?M3412 :: ?M3414) (?M3413 :: ?M3415)" with
 "All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) ls1) ls2 ls3".
Level 0: Backtrace:

In environment
A : Type
B : Type
P : list A → list A → A → A → B → Type
a : B
ls1 : list B
a0 : A
ls2 : list A
a1 : A
ls3 : list A
X0 : All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) (a :: ls1))
       ls2 ls3
X : P ls2 ls3 a0 a1 a
X2 : All (P ls2 ls3 a0 a1) ls1
Unable to unify "All2_fold ?M3411 (?M3412 :: ?M3414) (?M3413 :: ?M3415)" with
 "All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) ls1) ls2 ls3".