<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_add_on(self, add_on_code):
 """Return the `AddOn` for this plan with the given add-on code."""

        url = urljoin(self._url, '/add_ons/%s' % (add_on_code,))
        resp, elem = AddOn.element_for_url(url)
        return AddOn.from_element(elem)