Upon entering a function that is being traced,
*traced-arglist* is bound to the list of arguments given to the function.
*traced-arglist* is then printed after the function name in the output from tracing. It is accessible in the
:after forms to trace. However care should be used when manipulating this variable, since it is the value of
*traced-arglist* itself that is used when calling the traced function. Thus if this value is altered by the
:before forms then the function receives the altered argument list.
LispWorks User Guide and Reference Manual - 13 Feb 2015