docs: 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))