'Return the ``TreeSegmentWidget`` for the specified subtree.
:param path_to_tree: A list of indices i1, i2, ..., in, where
the desired widget is the widget corresponding to
``tree.children()[i1].children()[i2]....children()[in]``.
For the root, the path is ``()``.'

def collapsed_tree(self, *path_to_tree):

    return self._collapsed_trees[path_to_tree]
