Trace directives specify what debugging information is to be output at run-time.
TraceDirective =
"$trace" { FunctionName } ";" |
"$traceall" ";".
A directive $trace specifies that some debugging information is to be printed at the run time about the functions listed in the directive. When a function is called, its name is printed as well as the arguments passed. Then, when the call has been evaluated, the function name is printed as well as the results returned by the function.
A directive $traceall specifies that the debugging information is to be printed about all the functions whose definitions appear below the directive.