docs: def backtrace_root(node):
      """
      Example:
          >>> from utool.experimental.euler_tour_tree_avl import *  # NOQA
          >>> self = EulerTourTree(range(10))
          >>> self._assert_nodes()
          >>> root = self.root
          >>> node = self.get_node(5)
          >>> self.print_tree()
          >>> print('node = %r' % (node,))
          >>> rpath = backtrace_root(node)
          >>> print('rpath = %r' % (rpath,))
      """
      # Trace path to the root
      rpath = []
      prev = node
      now = node.parent
      while now is not None:
          if now.left is prev:
              rpath.append((now, 0))
          elif now.right is prev:
              rpath.append((now, 1))
          else:
              raise AssertionError('impossible state')
          prev = now
          now = now.parent
      return rpath