Greasemonkey: how to get noticed when new mail comes?

[Yongbin]

Howdy,

I'm writing some code in greasemonkey way, try to do some enhancement
of Gmail user interface.

In fact I want to run some scripts when user gets a new mail, but have
no idea to attach the new-mail event listener. I checked gmail-
greasemonkey-api, but there is nothing for this event.

So is there any good idea? Thanks!