Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ : list A
  Δ : list A
  H : All2_fold P Γ Δ
  ============================
   (All2_fold (λ (Γ Γ' : list A) (x y : A), P Γ' Γ y x) Δ Γ)


DEBUG:Going to execute:
induction H; constructor