pub type OrderedMap<K, V> = AVL<K, V>;
pub enum OrderedMap<K, V> { Empty, Node { key: Rc<K>, value: Rc<V>, left: Rc<AVL<K, V>>, right: Rc<AVL<K, V>>, }, }
key: Rc<K>
value: Rc<V>
left: Rc<AVL<K, V>>
right: Rc<AVL<K, V>>