':return: True if the URL was previously unknown'

@requires_setup
def add_url(self, url):

    if (not isinstance(url, URL)):
        msg = 'add_url    requires    a    URL    as    parameter    got    %s    instead.'
        raise TypeError((msg % type(url)))
    self._notify_observers(self.ADD_URL, url)
    return self.urls.add(url)
