usr/doc/lisp/tracedoc

 I have added a few things to the trace package and gotten some of the old
features to work.  There are still some loose ends to tie up however.

 You can now say
 (trace (append break))  which means that when append is called, first
        the trace enter line is printed, then  you enter a break loop.
        In this break loop you can test the value of the arguments to
        the called function using (arg n) where n is the arg number
        you want to test. of course you can also type in any lisp
        expression and it will be evaluated and its result printed.
        To exit this break loop, type ^D . At that point, the function
        will be evaluated and the trace exit line will be printed.
        This should work if append is compiled or interpreted.

You can also say
 (trace (append if (equal '(a b c) (arg 1))))
        will cause the trace enter and exit message to be printed only if
        the predicate is true.

You can also say
 (trace (append ifnot (equal 2 (arg nil))))
        which traces all calls to append which dont have two arguments.

you can even say
 (trace (append eval (print '"hi mom")))

        you can even use combinations of if, ifnot, eval and break if
        you dare but I havent quite fixed up their precedence.

        BUGS: the tricky stuff doesnt work if you trace nlambdas, and
        apply seems to fail on interpreted lambdas. trace should
        complain if you try to trace an already trace function.
        All of these bugs are minor and should disappear soon.