Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  a : B
  ls1 : list B
  a0 : A
  ls2 : list A
  a1 : A
  ls3 : list A
  X0 : All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) (a :: ls1))
         ls2 ls3
  X : P ls2 ls3 a0 a1 a
  X2 : All (P ls2 ls3 a0 a1) ls1
  ============================
   (All2_fold (λ (l1 l2 : list A) (x y : A), All (P l1 l2 x y) ls1) ls2 ls3)


DEBUG:Going to execute:
first [ is_ctor_list x | is_ctor_list y ]