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


DEBUG:Going to execute:
first
[ All2_All_swap_t_step
| let H1 := fresh in
  let H2 := fresh in
  eapply All_impl;
   [ eapply All_prod
   | intros ? [H1 H2]; constructor; (first [ exact H1 | exact H2 ]) ]; eauto;
   apply IH; clear IH ]
Level 0: Backtrace:

In environment
A : Type
B : Type
P : list A → list A → A → A → B → Type
Unable to unify "All ?M2932 (?M2933 :: ?M2934)" with
 "All
    (λ z : B, All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z) [] [])
    []".
Level 0: Backtrace:

In environment
A : Type
B : Type
P : list A → list A → A → A → B → Type
Unable to unify "All ?M2932 (?M2933 :: ?M2934)" with
 "All
    (λ z : B, All2_fold (λ (l1 l2 : list A) (x y : A), P l1 l2 x y z) [] [])
    []".