Lisp Macrology

Post crazy Lisp macros you've written in this thread.

I'll start off with a quick one I wrote 15minutes ago, PROGK:

>>13
What I mean is: generate code for k=0 that consists of the zeroth form in a SETF followed by the other forms, then follow that up with the same thing except the SETF wraps the oneth form, and so on. Then at runtime, when the value of k is actually known, jump to the proper version. By this means we could avoid having to do a comparison for every single form.