Do not perform an action twice in a session. Introduce three status bits
that every action has in a given session, in addition to the stamp:
KEEP-P (from previous session), DONE-P (and up-to-date, in this image),
NEED-P (needed in this image as part of this session).
BACKWARD INCOMPATIBILITY: Invert the meaning of NIL and T for STAMP,
so that NIL means out-of-date. This makes our code conceptually nicer,
and no one in Quicklisp uses our stamps, anyway. Still, beware.