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

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