code: public void enter(long orderId, Side side, long price, long size) {
      if (orders.containsKey(orderId)) {
        return;
      }
  
      if (side == Side.BUY) {
        buy(orderId, price, size);
      } else {
        sell(orderId, price, size);
      }
    }
docs: Enter an order to this order book.
  
  <p>The incoming order is first matched against resting orders in this order book. This
  operation results in zero or more Match events.
  
  <p>If the remaining quantity is not zero after the matching operation, the remaining quantity
  is added to this order book and an Add event is triggered.
  
  <p>If the order identifier is known, do nothing.
  
  @param orderId an order identifier
  @param side the side
  @param price the limit price
  @param size the size