<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _update_current(self):
 """ Makes sure that a current is set """

        if not self.current or self.current not in self.prefixes:
            if 'default' in self.prefixes:
                selected_current = 'default'
            elif self.prefixes:
                selected_current = sorted(self.prefixes.keys()).pop()
            else:
                # should never get here
                raise MalformedWorkdir(
                    'No current link and no prefixes in workdir %s' % self.path
                )

            logging.info(
                'Missing current link, setting it to %s',
                selected_current,
            )
            self._set_current(selected_current)