intros a b context der.
elim der; clear der.
intros t der_t.
apply Derivable_Intro with (Abs a t).
apply ImpIntro; assumption.
Qed.

Lemma derivable_vimp :
forall (context : flist) (l : list Int) (a b : form),
(forall context : flist, Derivable context a -> Derivable context b) ->
Derivable context (vimp l a) -> Derivable context (vimp l b).