Proof using.
intros.
find_apply_lem_hyp PB_net_defn.
intuition.
- destruct dst; subst; intuition.
- subst. right. right.
intuition. break_exists. intuition eauto.
Qed.

(* Specifications: *)
Lemma outputs_m_app :
forall tr1 tr2,
outputs_m (tr1 ++ tr2) = outputs_m tr1 ++ outputs_m tr2.