docs: def index(self, node: AbstractNode) -> int:
          """Return index of the node.
  
          If the node is not a child of this node, raise ``ValueError``.
          """
          if node in self.childNodes:
              return self.childNodes.index(node)
          elif isinstance(node, Text):
              for i, n in enumerate(self.childNodes):
                  # should consider multiple match?
                  if isinstance(n, Text) and n.data == node:
                      return i
          raise ValueError('node is not in this node')