def cancel_order(self, order_id):
        """Cancel an open order by ID (64 hexadecmial characters).

        :param order_id: Order ID.
        :type order_id: str | unicode
        :return: True if the order was cancelled successfully.
        :rtype: bool
        """
        self._log('cancel order {}'.format(order_id))
        result = self._rest_client.post(
            endpoint='/cancel_order',
            payload={'id': order_id}
        )
        return result == 'true'