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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 _set_current(self, new_current):
 """ Change the current default prefix, for internal usage Args: new_current(str):
 Name of the new current prefix, it must already exist Returns: None Raises: PrefixNotFound: if the given prefix name does not exist in the workdir """

        new_cur_full_path = self.join(new_current)
        if not os.path.exists(new_cur_full_path):
            raise PrefixNotFound(
                'Prefix "%s" does not exist in workdir %s' %
                (new_current, self.path)
            )

        if os.path.lexists(self.join('current')):
            os.unlink(self.join('current'))

        os.symlink(new_current, self.join('current'))
        self.current = new_current