[plt-scheme] Debugger: How much work to implement?

I'm curious as to how much work would be required to implement an
mzscheme debugger.  (I've had a look at MzTake, which is quite
impressive, but doesn't provide an obvious way to do some desirable
things.  I've also tried searching with Google, and found only a few
vague hints.)

For the purposes of this discussion, a basic debugger would have the
following features:

  1) An API for debugging a program, perhaps in another namespace (no
GUI required).
  2) Single-stepping.
  3) Enumeration of variables in scope at a given source location
(including values).
  4) Breakpoints.
  5) Support for ordinary debugging in the presence of macros.
  6) Evaluation of an expression in a given source location's scope.

How much of this exists today?  How much is trivial to implement?  It
would be desirable if the core features could run without MrEd.

Cheers,
Eric

http://iml.dartmouth.edu/tamale/