code: public long execute(long orderId, long quantity, long price) {
      Order order = orders.get(orderId);
      if (order == null) {
        return 0;
      }
  
      return execute(orderId, order, quantity, price);
    }
docs: Execute a quantity of an order in an order book. If the remaining quantity reaches zero, the
  order is deleted from the order book.
  
  <p>A Trade event and an update event are triggered.
  
  <p>If the order identifier is unknown, do nothing.
  
  @param orderId the order identifier
  @param quantity the executed quantity
  @param price the execution price
  @return the remaining quantity