code: public Node<K, V> add(Interval<K> i) {
        checkValidInterval(i);
        return add(new Node<K, V>(i));
     }
docs: Add the {@code Interval} into this {@code IntervalTree} and return the Node. Possible duplicates are found and the
  existing Node is returned instead of adding a new one.
  
  @param i an Interval to be inserted