... in cases such as inadvertent copyright violation...
it is of some legal importance that cancel really and truly
*destroy* the original message...
hmmm, supose the posting and its cancelation were to end
up on an incrementaly built CDRom, would this imply the
need to mask the posting in some clever manner? ...
Preliminary caution: I'm not a lawyer. Consult a professional before
doing anything rash. :-)
I believe that making a "best effort" which is likely to be reasonably
(if imperfectly) effective is the issue here. Cancels have never been
100% effective, but they're good enough to get rid of most readily-
accessible copies, which is enough to help mollify copyright holders
and reduce the chances that a lawsuit would be successful.
CDROM publishers might need to be a bit cautious about this kind of thing.
If I were doing it, I think I'd insert a bit of a delay to allow reasonably
prompt cancels to take effect. A court might find that since copyright
violations are known to occur and cancels are a reasonably effective response
to them, someone who plans to republish the stuff widely is obligated to
make reasonable efforts to preserve their effectiveness.