<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def backend(entry):
 """ Default URL shortener backend for Zinnia. """

    return '%s://%s%s' % (
        PROTOCOL, Site.objects.get_current().domain,
        reverse('zinnia:entry_shortlink', args=[base36(entry.pk)]))