def qop(self):
        """Indicates what "quality of protection" the client has applied to
        the message for HTTP digest auth."""
        def on_update(header_set):
            if not header_set and 'qop' in self:
                del self['qop']
            elif header_set:
                self['qop'] = header_set.to_header()
        return parse_set_header(self.get('qop'), on_update)
Indicates what "quality of protection" the client has applied to
        the message for HTTP digest auth.