Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  c : C
  a : C
  ls1 : list C
  IH : ∀ (ls2 : list A) (ls3 : list B),
         All (λ z : C, All2 (λ (x : A) (y : B), P x y z) ls2 ls3) (c :: ls1)
         → All2 (λ (x : A) (y : B), All (P x y) (c :: ls1)) ls2 ls3
  X1 : All (λ z : C, All2 (λ (x : A) (y : B), P x y z) [] []) (a :: ls1)
  ============================
   (All2 (λ (x : A) (y : B), All (P x y) (c :: a :: ls1)) [] [])


DEBUG:Going to execute:
progress subst
Level 0: Backtrace:

Failed to progress.
Level 0: Backtrace:

Failed to progress.