intros context context' a a' eq_context eq_a.
rewrite eq_context.
rewrite eq_a.
intros; assumption.
Qed.

Lemma derivable_subst :
forall (i : Int) (g : form) (context : flist) (a : form),
Derivable context a -> Derivable (subst_list i g context) (subst_form i g a).