intros a b context der_a der_ab.
elim der_a; clear der_a.
intros t der_t.
elim der_ab; clear der_ab.
intros s der_s.
apply Derivable_Intro with (App a s t).
apply ImpElim; assumption.
Qed.

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