Since the previous version of this code was first written, Racket's
requirements seem to have relaxed---but not completely!
If *all* related module paths are made relative, then the deps
test case breaks (but only the test is pre-compiled ?!!).
If requires from disappearing module(s) to residual module(s)
use absolute module paths, it seems to work.
Note: this version doesn't work with ~commit or ~!, because
it stores both choice points and undo actions in the failure
continuation. Commit and cut should discard choice points but
preserve undo actions.
