def find_flag_alias(self, flag):
        """Return alias set of a flag; return None if flag is not defined in
        "Options".
        """
        for each in self.opt_names:
            if flag in each:
                result = set(each)  # a copy
                result.remove(flag)
                return result
        return None
Return alias set of a flag; return None if flag is not defined in
        "Options".