add routing slip的用法和样例：
- US passenger carriers were expected to snap up the opportunity to fly to, or add routes to, China because international routes were more lucrative than domestic ones.
- According to the extendable simulation principles and methods of NS2, in this paper, adding routing protocols is studied and a QDR routing protocol in Mobile Ad Hoc Networks is added to NS2.
- More than once I've felt my heart slip a cog.
- I let it slip that I am expecting a baby.
- A few jokes will add a final fillip to your speech.