code: public void add(int index, E element) {
          checkPositionIndex(index);
  
          if (index == size)
              linkLast(element);
          else
              linkBefore(element, node(index));
      }
docs: Inserts the specified element at the specified position in this list.
  Shifts the element currently at that position (if any) and any
  subsequent elements to the right (adds one to their indices).
  
  @param index index at which the specified element is to be inserted
  @param element element to be inserted
  @throws IndexOutOfBoundsException {@inheritDoc}