def __getitem__(self, index):
if self._instance is not _unset and index not in self:<EOL><INDENT>self._instance[index]<EOL><DEDENT>return self._contents[index]<EOL>
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 `exceptions.LookupError`.