pub trait IStatTree {
// Required methods
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>;
}