def _to_alpha(self, num):
num = '<STR_LIT>'.join(c for c in text_type(num) if c in self._num_set)<EOL>if num:<EOL><INDENT>return num.translate(self._num_trans)<EOL><DEDENT>return '<STR_LIT>'<EOL>
Convert the Russell Index integer to an alphabetic string.

        This follows Robert C. Russell's Index algorithm, as described in
        :cite:`Russell:1917`.

        Parameters
        ----------
        num : int
            A Russell Index integer value

        Returns
        -------
        str
            The Russell Index as an alphabetic string

        Examples
        --------
        >>> pe = RussellIndex()
        >>> pe._to_alpha(3813428)
        'CRACDBR'
        >>> pe._to_alpha(715)
        'NAL'
        >>> pe._to_alpha(3614)
        'CMAD'