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