Upon entering a function that is being traced,
is bound to the list of arguments given to the function.
is then printed after the function name in the output from tracing. It is accessible in the
forms to trace. However care should be used when manipulating this variable, since it is the value of
itself that is used when calling the traced function. Thus if this value is altered by the
forms then the function receives the altered argument list.
LispWorks User Guide and Reference Manual - 21 Dec 2011