Translate a join node into latex qtree node.
        :param node: a treebrd node
        :return: a qtree subtree rooted at the node

def theta_join(self, node):
        """
        Translate a join node into latex qtree node.
        :param node: a treebrd node
        :return: a qtree subtree rooted at the node
        """
        return '[.${op}_{{{conditions}}}$ {left} {right} ]'\
            .format(op=latex_operator[node.operator],
                    conditions=node.conditions,
                    left=self.translate(node.left),
                    right=self.translate(node.right))