1. A method of tracing the activity of an expression, said method comprising the steps of: (a) specifying a machine-implemented process in which a trigger expression is to be traced; (b) specifying the trigger expression to be traced in the machine-implemented process; (c) storing the state of the trigger expression when it is active within the machine-implemented process without interrupting the process; and (d) restoring the state of the trigger expression when requested.