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
  a0 : All_fold P Δ
  b : All_fold (λ Γ : list A, P (Γ ++ Δ)) Γ
  ============================
   (All_fold P Δ)


DEBUG:Going to execute:
fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]