Some functions (usually applied in single-sided conditions) return boolean values (values that have only two states and are represented only by TRUE or FALSE, 0 or 1, depending on the veracity of the condition at the moment of verification). There is the implicit mode of specifying that some single-sided condition is true just by adding the exclamation mark at the end, like this:
last_order_type(BUY_OPEN)!. This is the easiest and shortest method of telling the expert the condition must be true in order to trigger a signal. However, there is the longer way which, in addition to define that conditions must be TRUE, it also allows setting the inverse, that is, to define that conditions must be FALSE.
|This condition will trigger an event signal only if the last order event was the opening of a buy order.|
|This condition will trigger an event signal only if the last order event was NOT the opening of a buy order.|