Goal:
    A : Type  P : list A → A → Type  Γ : list A  Δ : list A
  ============================
   (All_fold (λ Γ : list A, P (Γ ++ Δ)) Γ
    → All_fold P Δ → All_fold P (Γ ++ Δ))


DEBUG:Going to execute:
induction 1; simpl