TraceOn is an option for Trace and related functions which specifies when tracing should be switched on.
With the setting TraceOn -> patt, Trace and related functions do not start tracing until they encounter expressions to evaluate which match the pattern patt. This pattern is also tested against tags associated with the evaluation.
TraceOff can be used within tracing switched on by TraceOn.
Once tracing has been switched off by TraceOff, however, TraceOn will not switch it on again.
During the execution of Trace, the settings for TraceOn and TraceOff can be modified by resetting the values of the global variables $TraceOn and $TraceOff.
See The Mathematica Book: Section 2.5.10.
See also: TraceOff.
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