code: public void add(long instrument, long orderId, Side side, long price, long size) {
      if (orders.containsKey(orderId)) {
        return;
      }
  
      OrderBook book = books.get(instrument);
      if (book == null) {
        return;
      }
  
      Order order = new Order(book, side, price, size);
  
      boolean bbo = book.add(side, price, size);
  
      orders.put(orderId, order);
  
      listener.update(book, bbo);
    }
docs: Add an order to an order book.
  
  <p>An update event is triggered.
  
  <p>If the order book for the instrument is closed or the order identifier is known, do nothing.
  
  @param instrument the instrument
  @param orderId the order identifier
  @param side the side
  @param price the price
  @param size the size