intros context a b der_a der_b.
elim der_b; clear der_b.
intros t der_b.
apply (derivable_cut_aux (a :: context) t b der_b a fnil context).
trivial.
assumption.
Qed.

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