'Called when a the child with \'idx\' has moved behind \'pivot\' by
\'diff\'.'

def _child_has_moved(self, idx, pivot, diff):

    self._end.move(pivot, diff)
    for child in self._children[(idx + 1):]:
        child._move(pivot, diff)
    if self._parent:
        self._parent._child_has_moved(self._parent._children.index(self), pivot, diff)
