If a frame mutates a global and then signals, taking a retry re-runs the
mutation. Control resumes at the restart-case and runs forward; nothing is
undone. Common Lisp has the same property and offers no help either, so this
is written down rather than fixed.
The discipline is that the author chooses where the retry boundary is: a
restart-case above the mutations re-runs them, one below re-runs only what
follows. Put the restart before anything mutates, make the retried section
idempotent, or snapshot what will be re-applied.
It matters more here than in most Lisps because the intended use is a game
loop, and a bad index signalling BoundsError rather than ending the process
made abandoning and retrying a frame an ordinary thing to do.
conditions.org and web/index.html already carried the mechanical half as a
one-line gotcha; those are rewritten in place rather than gaining a second
bullet beside them. spec-conditions.md takes it in section 5, which already
enumerates what a transfer does and does not do. No numbered case changed
meaning.
Joseph Ferano
042b2ce4d0