Discussion of article "Library for easy and quick development of MetaTrader programs (part XXI): Trading classes - Base cross-platform trading object"

It is nice to have an easy access to various data at any time. However, that data is useless if we cannot apply it in trading. This means we need trading functionality along with the already existing one. 
This section will be relatively large, and we will do everything step by step.

• We should be able to send any trading requests from any platform, be it MetaTrader 5 or MetaTrader 4, without even thinking about differences between them. Everything should be unified.
• First, we need to verify trading requests in order not to load the server with deliberately erroneous requests.
  
• We need to consider and correctly handle the return codes of the trade server. What does an EA do while sending a request to the server? It maintains the 'request-response' dialog with the server. Our task is to correctly arrange such a "communication channel", i.e. create the methods of handling trade server responses.
• We need to create several options of handling server responses since sometimes we need to open a position "preferably at any cost". To do this, we need to arrange a repeated sending of a request to the server in case of a refusal to place an order — we can either adjust the trading request parameters or re-send it, or leave all the parameters intact but wait for the right moment when the request with these parameters is passed to send it immediately. Besides, we need to consider the price level in order not to re-send an order at a knowingly worse price.
  Sometimes, we need to send a trading request and continue work regardless of a request result.
  
• Besides, we need to arrange the work with trading classes so that to avoid issues when placing a library-based program to MQL5 Market. The program should pass all the checks smoothly.