Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  b : B
  a : A
  ls3 : list A
  ============================
   (All
      (λ z : B,
         All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z) []
           (a :: ls3)) [b]
    → All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) [b]) []
        (a :: ls3))


DEBUG:Going to execute:
first
[ All2_All_swap_t_step
| let H1 := fresh in
  let H2 := fresh in
  eapply All2_fold_impl;
   [ eapply All2_fold_prod
   | intros * [H1 H2]; constructor; (first [ exact H1 | exact H2 ]) ]
| let H1 := fresh in
  let H2 := fresh in
  eapply All2_fold_impl; [ eapply IH |  ]; clear IH ]
Level 0: Backtrace:

In environment
A : Type
B : Type
P : list A → list A → A → A → B → Type
b : B
Unable to unify "All2_fold ?M2995 (?M2996 :: ?M2998) (?M2997 :: ?M2999)" with
 "All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) [b]) [] []".
Level 0: Backtrace:

In environment
A : Type
B : Type
P : list A → list A → A → A → B → Type
b : B
Unable to unify "All2_fold ?M2995 (?M2996 :: ?M2998) (?M2997 :: ?M2999)" with
 "All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) [b]) [] []".