Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  c : C
  b : B
  ls3 : list B
  ============================
   (All (λ z : C, All2 (λ (x : A) (y : B), P x y z) [] (b :: ls3)) [c]
    → All2 (λ (x : A) (y : B), All (P x y) [c]) [] (b :: ls3))


DEBUG:Going to execute:
first
[ All2_All_swap_t_step
| let H1 := fresh in
  let H2 := fresh in
  eapply All2_impl;
   [ eapply All2_prod
   | intros * [H1 H2]; constructor; (first [ exact H1 | exact H2 ]) ]
| let H1 := fresh in
  let H2 := fresh in
  eapply All2_impl; [ eapply IH |  ]; clear IH ]
Level 0: Backtrace:

In environment
A : Type
B : Type
C : Type
P : A → B → C → Type
c : C
Unable to unify "All2 ?M2988 (?M2989 :: ?M2991) (?M2990 :: ?M2992)" with
 "All2 (λ (x : A) (y : B), All (P x y) [c]) [] []".
Level 0: Backtrace:

In environment
A : Type
B : Type
C : Type
P : A → B → C → Type
c : C
Unable to unify "All2 ?M2988 (?M2989 :: ?M2991) (?M2990 :: ?M2992)" with
 "All2 (λ (x : A) (y : B), All (P x y) [c]) [] []".