def node_pairing(self):
        """if "node" then test current node and next one
        if "tag", then create tests for every pair of the current tag.
        """
        value = self.attributes['node_pairing']
        if value not in IMB.NODE_PAIRING:
            msg = 'Unexpected {0} value: got "{1}" but valid values are {2}'
            msg = msg.format('node_pairing', value, IMB.NODE_PAIRING)
            raise ValueError(msg)
        return value