Internally every log statement that is allowed to enter the system (passes initial Logger filtering) is a IMessage implementation.
Every Logger can be instantiated with a specific MessageFactory which will take care of creating internal message used by the system. For example if you're logging na Orange object often you might consider creating your own message factory implementation which will produce OrangeMessages where you can control exactly how the Orange object will be serialized to the output.
Message takes Expr in constructor and uses Printer to output pretty printed expression.