intros context1 context2 a der1.
elim context2; clear context2.
assumption.
intros b context2 ih.
simpl in |- *; apply derivable_weak; assumption.
Qed.

Lemma derivable_cut_aux :
forall (context : flist) (t : proof_term) (b : form),
derives context t b ->
forall (a : form) (l1 l2 : flist),
context = l1 ++ a :: l2 -> Derivable fnil a -> Derivable (l1 ++ l2) b.