code: public T removeFirst() {
          try {
              modifyLock.lock();
              Iterator<T> it = iterator();
              if (it.hasNext()) {
                  T item = it.next();
                  it.remove();
                  return item;
              }
          } finally {
              modifyLock.unlock();
          }
          return null;
      }
docs: Special method that removes head element.
  @return