Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Type
  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) ls1
         → (ls1 = []) + All2 (λ (x : A) (y : B), All (P x y) ls1) ls2 ls3
  b : B
  ls3 : list B
  x : C
  l : list C
  X0 : All2 (λ (x : A) (y : B), P x y a) [] (b :: ls3)
  X1 : All (λ z : C, All2 (λ (x : A) (y : B), P x y z) [] (b :: ls3)) ls1
  H0 : x = a
  H1 : l = ls1
  ============================
   ((a :: ls1 = []) +
    All2 (λ (x : A) (y : B), All (P x y) (a :: ls1)) [] (b :: ls3))%type


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

Failed to progress.
Level 0: Backtrace:

Failed to progress.