pub trait IStatTree {
    fn sum_subtree_child_count(&self) -> Option<u64>;
    fn node_degree(&self) -> Option<u64>;
    fn max_depth(&self) -> Option<u64>;
    fn mean_depth(&self) -> Option<u64>;
    fn min_depth(&self) -> Option<u64>;
    fn balance_ratio(&self) -> Option<f64>;
}