Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  c : C
  a : A
  ls2 : list A
  b : B
  ls3 : list B
  X : P a b c
  X2 : All2 (λ (x : A) (y : B), P x y c) ls2 ls3
  ============================
   (All2 (λ (x : A) (y : B), All (P x y) [c]) ls2 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
a : A
ls2 : list A
b : B
ls3 : list B
X : P a b c
X2 : All2 (λ (x : A) (y : B), P x y c) ls2 ls3
Unable to unify "All ?M3118 (?M3119 :: ?M3120)" with 
"All (P a b) []".
Level 0: Backtrace:

In environment
A : Type
B : Type
C : Type
P : A → B → C → Type
c : C
a : A
ls2 : list A
b : B
ls3 : list B
X : P a b c
X2 : All2 (λ (x : A) (y : B), P x y c) ls2 ls3
Unable to unify "All ?M3118 (?M3119 :: ?M3120)" with 
"All (P a b) []".