Challenge 2: classify/2 — Green or Red Cuts? — Possible Solution
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classify_demo.pl:

classify(X, negative) :- X < 0, !.
classify(X, zero)     :- X =:= 0, !.
classify(X, positive).

Queries:

    ?- classify(-5, R).
    R = negative.

    ?- classify(0, R).
    R = zero.

    ?- classify(7, R).
    R = positive.

-- Are these cuts green or red?
--
-- Both cuts here are RED cuts, not green ones. Without the first
-- cut, classify(-5, R) would still correctly produce R = negative
-- from clause 1 -- but backtracking (via ;) would then ALSO try
-- clause 2 (X =:= 0, which fails for -5, so nothing extra happens
-- there) and clause 3, which has NO guard at all and would
-- incorrectly ALSO succeed with R = positive for -5. Without the
-- cuts, classify(-5, R) would report TWO different answers
-- (negative AND positive) instead of just one -- a genuinely
-- different, WRONG set of solutions. Since removing the cuts
-- changes which solutions come back (not just how many redundant
-- alternatives get tried), these are red cuts by the chapter's own
-- definition -- they are load-bearing for correctness, not just an
-- efficiency optimization.

WHY THIS WORKS AS AN ANSWER
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This correctly identifies both cuts as red rather than green by
applying the chapter's own test -- checking whether removing them
would change the actual set of solutions returned, not just how many
alternatives are explored -- and demonstrates the real, wrong extra
answer that would appear without them.