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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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)