code: public boolean add(long seqno, T element) {
          lock.lock();
          try {
              return _add(seqno, element, true, null);
          }
          finally {
              lock.unlock();
          }
      }
docs: Adds an element if the element at the given index is null. Returns true if no element existed at the given index,
  else returns false and doesn't set the element.
  @param seqno
  @param element
  @return True if the element at the computed index was null, else false