in src/tree.rs [381:389]
fn set_children(&mut self, parent: &mut TreeNode<P>, lref: usize, rref: usize) {
parent.set_lch(lref);
parent.set_rch(rref);
let lch = self.nodes[lref].get_value();
let rch = self.nodes[rref].get_value();
let value = Mergeable::merge(lch, rch);
parent.set_value(value);
}