'Retrieve the child tree one level down at the given ``index``.
If the index is not in the instance that this tree corresponds to and
is not known by this tree, whatever error would be raised by
``instance.__getitem__`` will be propagated (usually this is some
subclass of :class:`LookupError`.'

def __getitem__(self, index):

    if ((self._instance is not _unset) and (index not in self)):
        self._instance[index]
    return self._contents[index]
