code: def index(self, item):
          
          index_at = None
          for (i, child) in enumerate(self._children):
              if child.item == item:
                  index_at = i
                  break
          if index_at is None:
              raise ValueError("%s is not contained in any child" % (item))
          return index_at
docs: Finds the child index of a given item, searchs in added order.