msg_trace(-OldValue, +OnOrOff)

The predicate msg_trace/2 returns the current value of the message-trace flag (on or off) and resets its value. The message-trace flag has one of the values on or off. OldValue is bound to the previous value of the flag, and the flag is reset to the value of OnOrOff, which must be either on or off. The call msg_trace(X,X) returns the current value without changing it. When the message-trace flag is on, messages to and from the servant are traced. By executing call_servant(msg_trace(_,on)), tracing can be turned on in the servant.