Function is23

src/tree/binary/rb2.rs:402–414

Does the tree have no red right links, and at most one (left) red links in a row on any path?

(root: Option<NonNull<Node<K, V>>>, x: Option<NonNull<Node<K, V>>>)

/// Does the tree have no red right links, and at most one (left)
/// red links in a row on any path?
fn is23<K, V>(root: Option<NonNull<Node<K, V>>>, x: Option<NonNull<Node<K, V>>>) -> bool {
    if x.is_none() {
        true
    } else {
        let x = NodeQuery::new(x);
        let root = NodeQuery::new(root);
        if x.right().is_red() || (x.node != root.node && x.is_red() && x.left().is_red()) {
            false
        } else {
            is23(root.node, x.left().node) && is23(root.node, x.right().node)
        }
    }
}

fn calc_blacks<K, V>(x: Option<NonNull<Node<K, V>>>) -> usize {
    let mut x = NodeQuery::new(x);