trace and untrace control the invocation of the trace facility.
Invoking trace with one or more function-names causes the denoted functions to be "traced". Whenever a traced function is invoked, information about the call, about the arguments passed, and about any eventually returned values is printed to trace output. If trace is used with no function-names, no tracing action is performed; instead, a list of the functions currently being traced is returned.
Invoking untrace with one or more function names causes those functions to be "untraced" (i.e. no longer traced).
If untrace is used with no function-names, all functions currently being traced are untraced.
1 Enter FACT 3
1 Exit FACT 6
Of course, the format of traced output is implementation-dependent.
Might change the definitions of the functions named by function-names.
Whether the functions named are defined or already being traced.
Tracing an already traced function, or untracing a function not currently being traced, should produce no harmful effects, but might signal a warning.
Although trace can be extended to permit non-standard options, implementations are nevertheless encouraged (but not required) to warn about the use of syntax or options that are neither specified by this standard nor added as an extension by the implementation, since they could be symptomatic of typographical errors or of reliance on features supported in implementations other than the current implementation.