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

def assign(self, node):
        """
        Translate an assign node into latex qtree node.
        :param node: a treebrd node
        :return: a qtree subtree rooted at the node
        """
        child = self.translate(node.child)
        attributes = ''
        if node.attributes:
            attributes = '({})'.format(','.join(node.attributes.names))
        return '[.${name}{attributes}$ {child} ]'\
            .format(name=node.name, attributes=attributes, child=child)