code: private Node<K, V> cutOldestChild(Node<K, V> n) {
          Node<K, V> oldestChild = n.o_c;
          if (oldestChild != null) {
              if (oldestChild.y_s != null) {
                  oldestChild.y_s.o_s = n;
              }
              n.o_c = oldestChild.y_s;
              oldestChild.y_s = null;
              oldestChild.o_s = null;
          }
          return oldestChild;
      }
docs: Cut the oldest child of a node.
  
  @param n
  the node
  @return the oldest child of a node or null