code: private Node<K, V> link(Node<K, V> y, Node<K, V> x) {
          y.parent = x;
          Node<K, V> child = x.child;
          if (child == null) {
              x.child = y;
              y.next = y;
              y.prev = y;
          } else {
              y.prev = child;
              y.next = child.next;
              child.next = y;
              y.next.prev = y;
          }
          return x;
      }
docs: /*
  (unfair) Link y as a child of x.