docs: 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)