code: public AbstractJournalOperation removeFirst()
      {
          AbstractJournalOperation op = head;
          if (op == null)
              throw new NoSuchElementException();
          
          head = op.next();
          if (head == null)
              tail = null;
          op.setNext(null);
          size--;
          
          return op;
      }
docs: Remove the first available journal operation in queue
  @return a journal operation
  @throws NoSuchElementException if queue is empty