intuition.
unfold negligible in *.
intuition.
edestruct H0.
econstructor.
intuition.
eapply H1.
eauto.
rewrite H3.
eauto.
Qed.

Lemma negligible_plus :
forall f1 f2,
negligible f1 ->
negligible f2 ->
negligible (fun n => f1 n + f2 n)%rat.