MQL4 Reference 
Trading functions
A group of functions intended for trading management.
Trading functions of OrderSend(), OrderClose,
OrderCloseBy, OrderDelete, and OrderModify
cannot be called from custom indicators.
Trading functions can be used in experts and scripts. Trading functions can be called only if the "Allow live trading" property of this particular expert is checked.
To trade from experts and scripts, only one thread was provided that was launched in the program trade context (context of
automated trading from experts and scripts). This is why, if this context is occupied with an expert trading operation,
another expert or script cannot call trading functions at that moment due to error 146 (ERR_TRADE_CONTEXT_BUSY).
To check whether it is possible to trade or not, one has to use the IsTradeAllowed() function.
To make a clear sharing of access to the trading context, one can use a semaphore on the basis of a global variable
the value of which must be changed using the GlobalVariableSetOnCondition() function.
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