Goal:
    A : Type  B : Type  C : Type  P : A → B → C → Type  b : B  ls3 : list B
  ============================
   (All2 (λ (x : A) (y : B), All (P x y) []) [] (b :: ls3)
    → All (λ z : C, All2 (λ (x : A) (y : B), P x y z) [] (b :: 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
C : Type
P : A → B → C → Type
Unable to unify "All ?M2932 (?M2933 :: ?M2934)" with
 "All (λ z : C, All2 (λ (x : A) (y : B), P x y z) [] []) []".
Level 0: Backtrace:

In environment
A : Type
B : Type
C : Type
P : A → B → C → Type
Unable to unify "All ?M2932 (?M2933 :: ?M2934)" with
 "All (λ z : C, All2 (λ (x : A) (y : B), P x y z) [] []) []".