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


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