Re: Lisp debugger problems.

[Richard Stallman]

    IIRC defadvice has some support for such things (e.g. you can add an advice
    even before the function is defined).  Maybe it'd be a good idea to
    implement debug.el in terms of advice.

Right now, the code to implement debug on entry is so simple that it
would be a shame to use something as heavyweight as the advice
mechanism.  However, if these new changes add a lot of complexity,
maybe using the advice mechanism will become the simplest way.