tree_cursor.cpp

std::optional<lager::cursor<Volume>> volumeCursorAt(
        lager::cursor<VolumeTree> treeCursor, VolumeTree const &tree, int &index)
{
    if (index == 0) {
        return {treeCursor[&VolumeTree::volume]};
    } else {
        size_t i = 0;
        for (auto const &child : tree.children) {
            --index;
            std::optional<lager::cursor<Volume>> res =
                volumeCursorAt(
                    treeCursor[&VolumeTree::children][i][&VolumeVolumeRegistration::tree],
                    child.tree,
                    index);
            if (res.has_value()) {
                return res;
            }
            ++i;
        }
        return std::nullopt;
    }
}