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


DEBUG:Going to execute:
induction H; constructor