Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Q : list A → list A → A → A → Type
  Γ : list A
  Δ : list A
  H : All2_fold P Γ Δ
  HP : ∀ (Γ Δ : list A) (T U : A), #|Γ| = #|Δ| → P Γ Δ T U → Q Γ Δ T U
  H0 : #|Γ| = #|Δ|
  ============================
   (All2_fold Q Γ Δ)


DEBUG:Going to execute:
induction H; constructor