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

Lemma co_subst_fresh :
forall G phi a0 x s,
PropWff G phi -> Ctx ((x ~ s) ++ G) -> tm_subst_tm_constraint a0 x phi = phi.