Goal:
  
  A : Type
  P : list A → A → Type
  a : A
  Γ : list A
  Δ : list A
  X : All_fold P (Γ ++ Δ)
  p : P (Γ ++ Δ) a
  X0 : All_fold P Δ × All_fold (λ Γ : list A, P (Γ ++ Δ)) Γ
  ============================
   (All_fold P Δ → All_fold (λ Γ : list A, P (Γ ++ Δ)) Γ → All_fold P Δ)


DEBUG:Going to execute:
intro