The trace facility is a debugging aid enabling you to follow the execution of particular functions. At any time there are a set of functions (and macros and methods) which are being monitored in this way. The normal behavior when a call is made to one of these functions is for the function's name, arguments and results to be printed out by the system. More generally you can specify that particular forms should be executed before or after entering a function, or that certain calls to the function should cause it to enter the main debugger. Tracing of a function continues even if the function is redefined; however the tracing of some structure accessors and so forth may be lost if the compiler is set to optimize the code for efficiency (so that these calls are inlined).
The standard way of getting functions to be traced in this way is to call the macro trace with the symbols of the functions (or macros or generic functions) concerned. In addition it is possible to restrict tracing to a particular method (rather than a generic function), by specifying the requisite classes for the arguments in the call to trace. The trace facility handles recursive and nested calls to the functions concerned.