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(dynamic-wind BEFORE THUNK AFTER)
- Calls BEFORE; runs THUNK; calls AFTER; returns THUNK's value.
- If THUNK raises, AFTER still runs before the raise propagates.
- Implementation: outcome-sentinel pattern (same trick as guard
and with-exception-handler) — catch THUNK's raise inside a
host guard, run AFTER unconditionally, then either return the
value or re-raise outside the catch.
Not implemented: call/cc-escape tracking. R7RS specifies that
dynamic-wind's BEFORE and AFTER thunks should re-run when control
re-enters or exits the dynamic extent via continuations. That
requires explicit dynamic-extent stack tracking, deferred until
a consumer needs it (probably never needed for pure-eval Scheme
programs; matters for first-class-continuation-heavy code).
5 tests: success ordering, return value, after-on-raise,
raise propagation, nested wind.
237 total Scheme tests now (62 + 23 + 49 + 78 + 25).