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);
    }