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

Lemma tm_subst_co_fresh_1 :
forall G a A a0 c s,
Typing G a A -> Ctx ((c ~ s) ++ G) -> co_subst_co_tm a0 c A = A.