exact I.
split.
constructor;apply H;in_solve.
apply IHΔ;intros;apply H;in_solve.
Qed.

(* Specifications: *)
Lemma Nc_list_app : forall Γ Δ1 Δ2, Γ ⊢⊢ Δ1 -> Γ ⊢⊢ Δ2 -> Γ ⊢⊢ Δ1++Δ2.
intros;induction Δ1.