int start()
  {
    // check whether the market should be entered now
    ...
    // calculate the Stop Loss and Take Profit levels, and lot size
    ...
    // check whether trade context is free
    int TradeAllow = _IsTradeAllowed();
    if(TradeAllow < 0) 
      { 
        return(-1); 
      }
    if(TradeAllow == 0)
      {
        RefreshRates();
        // recalculate the Take Profit and Stop Loss levels
        ...
      }
    // open a position
    if(OrderSend(...) < 0) 
        Alert("Error opening position # ", GetLastError());
    return(0);
  }

/////////////////////////////////////////////////////////////////////////////////
// int _IsTradeAllowed( int MaxWaiting_sec = 30 )
//
// the function checks the trade context status. Return codes:
//  1 - trade context is free, trade allowed
//  0 - trade context was busy, but became free. Trade is allowed only after 
//      the market info has been refreshed.
// -1 - trade context is busy, waiting interrupted by the user (expert was removed from 
//      the chart, terminal was shut down, the chart period and/or symbol was changed, etc.)
// -2 - trade context is busy, the waiting limit is reached (MaxWaiting_sec). 
//      Possibly, the expert is not allowed to trade (checkbox "Allow live trading" 
//      in the expert settings).
//
// MaxWaiting_sec - time (in seconds) within which the function will wait 
// until the trade context is free (if it is busy). By default,30.
/////////////////////////////////////////////////////////////////////////////////
int _IsTradeAllowed(int MaxWaiting_sec = 30)
  {
    // check whether the trade context is free
    if(!IsTradeAllowed())
      {
        int StartWaitingTime = GetTickCount();
        Print("Trade context is busy! Wait until it is free...");
        // infinite loop
        while(true)
          {
            // if the expert was terminated by the user, stop operation
            if(IsStopped()) 
              { 
                Print("The expert was terminated by the user!"); 
                return(-1); 
              }
            // if the waiting time exceeds the time specified in the 
            // MaxWaiting_sec variable, stop operation, as well
            if(GetTickCount() - StartWaitingTime > MaxWaiting_sec * 1000)
              {
                Print("The waiting limit exceeded (" + MaxWaiting_sec + " sec.)!");
                return(-2);
              }
            // if the trade context has become free,
            if(IsTradeAllowed())
              {
                Print("Trade context has become free!");
                return(0);
              }
            // if no loop breaking condition has been met, "wait" for 0.1 
            // second and then restart checking            Sleep(100);
          }
      }
    else
      {
        Print("Trade context is free!");
        return(1);
      }
  }