Where have 'ui-router' included in tabs example

in some tutorial to be able to user ui-router we need to include ‘ui-router’ in module when we first created.
angular.module(‘starter’, [

but in the sample tabs I don’t see ui-router mentioned in the module, but still this example works well.

and I wonder where this tag has been included? ‘ui-router’, and when I try to put it in, app is error.

/Pherum Un