<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def keyserver(self):
 """ The preferred key server specified in this signature, if any. Otherwise, an empty ``str``. """

        if 'PreferredKeyServer' in self._signature.subpackets:
            return next(iter(self._signature.subpackets['h_PreferredKeyServer'])).uri
        return ''