Module ext_subst (weak : ext_weak_sig) <: ext_subst_sig.
Include weak.
induction G2; [ | inversion 1]; simpl; auto.
Qed.

Lemma tm_subst_fresh_1 :
forall G a A a0 x s,
Typing G a A -> Ctx ((x ~ s) ++ G) -> tm_subst_tm_tm a0 x A = A.