Exercise 2: A Second New Discount Type — Possible Solution
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THE NEW CLASS
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    class SilverDiscount(DiscountStrategy):
        def apply(self, price): return price * 0.85

Follows the same pattern as this chapter's own PlatinumDiscount - a
new subclass of DiscountStrategy, nothing else touched.

VERIFYING ALL FIVE EXISTING CLASSES STAYED UNCHANGED
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    PricingEngine unchanged: True
    DiscountStrategy unchanged: True
    NoDiscount unchanged: True
    GoldDiscount unchanged: True
    PlatinumDiscount unchanged: True

Every one of the five classes that existed before this exercise's own
extension - including PlatinumDiscount, which was itself the "new"
class in this chapter's own original example - is confirmed byte-
identical afterward. Extending the hierarchy a second time didn't
require touching the first extension either.

VERIFYING THE NEW DISCOUNT WORKS CORRECTLY
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    PricingEngine correctly applies the new SilverDiscount: 85.0 (expected 85.0)

PricingEngine(SilverDiscount()).calculate(100) correctly returns 85.0
- and PricingEngine's own source was one of the five confirmed
unchanged above.

WHY THIS CONFIRMS OPEN/CLOSED HOLDS FOR REPEATED EXTENSION, NOT JUST ONE
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This chapter's own finding showed the hierarchy staying closed for ONE
extension (adding Platinum). This exercise confirms the SAME guarantee
holds for a SECOND, independent extension (adding Silver) - and,
critically, that the first extension doesn't need to be touched or
even re-read to add the second one. Each new discount type is a fully
self-contained addition; the hierarchy's own "closed for modification"
property doesn't erode as more extensions accumulate.

WHY THIS WORKS AS AN ANSWER
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A second, genuinely new discount class is added using this chapter's
own established pattern, all five now-existing classes (not just the
originally-tested four) are verified unchanged, and the new discount's
own correct behavior is verified through the confirmed-unmodified
PricingEngine.