General help needed for using Closure's advanced compression when my scripts depend on Backbone.js and Underscore.js.

I've tried reading through Closure's API tutorial, but couldn't get my compiled scripts working. Specifically I added a js\_externs parameter:

```
('js_externs', 'function _(){};function Backbone(){}')

```

into my python script (a la <http://code.google.com/closure/compiler/docs/api-tutorial3.html#howto-app>) but this hasn't worked.