Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  P' : list A → list A → A → A → Type
  Γ : list A
  Δ : list A
  cr : All2_fold P Γ Δ
  Hcr : ∀ (Γ Δ : list A) (d d' : A),
          All2_fold P Γ Δ → All2_fold P' Γ Δ → P Γ Δ d d' → P' Γ Δ d d'
  ============================
   (All2_fold P' Γ Δ)


DEBUG:Going to execute:
induction cr; constructor