given a token node ID, returns the token unicode string.

        Parameters
        ----------
        token_node_id : str
            the ID of the token node
        token_attrib : str
            name of the node attribute that contains the token string as its
            value (default: token).

        Returns
        -------
        token : unicode
            the token string

def get_token(self, token_node_id, token_attrib='token'):
        """
        given a token node ID, returns the token unicode string.

        Parameters
        ----------
        token_node_id : str
            the ID of the token node
        token_attrib : str
            name of the node attribute that contains the token string as its
            value (default: token).

        Returns
        -------
        token : unicode
            the token string
        """
        return self.node[token_node_id][self.ns+':'+token_attrib]