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

Lemma derivable_a_a_imp_b__derivable_b :
forall (a b : form) (context : flist),
Derivable context a -> Derivable context (Imp a b) -> Derivable context b.