Exercise 2: A New Loyalty Discount Tier — Possible Solution
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THE NEW CLASS
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    class LoyaltyDiscount(DiscountStrategy):
        def apply(self, total): return total * 0.85   # 15% off

Added to the chapter's own final integrated DiscountStrategy hierarchy,
alongside NoDiscount and GoldDiscount.

RESULTS
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    NoDiscount unchanged: True
    GoldDiscount unchanged: True

    LoyaltyDiscount + StandardShipping checkout total: $39.00
    Hand calculation: 40 * 0.85 + 5 = $39.00
    Match: True

Both pre-existing discount classes are confirmed byte-identical after
adding LoyaltyDiscount, and a full checkout using the new tier produces
the correct total, matching a hand calculation exactly.

WHY THIS WORKS AS AN ANSWER
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This reproduces Step 4's own byte-identical-classes finding one more
time, now against the chapter's own final, fully integrated system
rather than the standalone strategy-hierarchy demonstration earlier in
the chapter - confirming the Open/Closed guarantee holds all the way
through to the assembled system, not just in the isolated step where
it was first established.