Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  HP : ∀ (Δ : list A) (x : A), P Δ Δ x x
  ============================
   (∀ Δ : list A, All2_fold P Δ Δ)


DEBUG:Going to execute:
induction Δ; constructor