This function returns the single most frequent calculated sentiment
        expressed by all stored users or by a list of specified users as a
        dictionary.

def users_sentiments_single_most_frequent(
        self,
        usernames = None,
        ):
        """
        This function returns the single most frequent calculated sentiment
        expressed by all stored users or by a list of specified users as a
        dictionary.
        """
        users_sentiments_single_most_frequent = dict()
        if usernames is None:
            usernames = self.usernames()
        try:
            for username in usernames:
                sentiment = self.user_sentiments_most_frequent(
                    username             = username,
                    single_most_frequent = True
                )
                users_sentiments_single_most_frequent[username] = sentiment
            return users_sentiments_single_most_frequent
        except:
            log.error("error -- possibly a problem with tweets stored")
            return None