[plt-scheme] Can I create an event that's like an alarm, but can be reset?

Yikes!  Thanks for doing my homework for me :-)

Seriously, thanks; I've already written my own thing which I'm happy
with, and which I'll show anyone who's interested.  (It looks like you
and I approached it pretty much the same way, although I'd describe
mine as "simpler but cruder").
-- 
There is such a thing as American-ness.  ... [H]ackers ... come
closest of any group I know to embodying it.  Closer, probably, than
the men running our government.
        -- Paul Graham