For Django 1.3.1, where is the source code that makes it possible to do a `from django.conf.urls import patterns` ?

In the [Django 1.3.1 source code for the django.conf.urls package](https://code.djangoproject.com/browser/django/tags/releases/1.3.1/django/conf/urls), `__init__.py` is empty, so there is no code or `__all__` variable. The code for the `patterns()` function appears to be in `defaults.py` which does have an `__all__` variable. 

How does Python end up including a `pattern()` function in the django.conf.urls package?