Exercise 2: Raising the Bulk-Discount Threshold — Possible Solution
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THE CHANGE
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    # before
    def apply_bulk_discount(total):
        return total - 5 if total > 100 else total

    # after
    def apply_bulk_discount(total):
        return total - 5 if total > 150 else total

Only the comparison value (100 -> 150) changes, inside apply_bulk_
discount() specifically - this time the isolated function under test is
a DIFFERENT one than this chapter's own platinum-tier example.

VERIFYING THE OTHER TWO FUNCTIONS STAYED UNTOUCHED
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    apply_tier_discount unchanged: True
    apply_express_fee unchanged: True

Both apply_tier_discount() and apply_express_fee() - captured via
inspect.getsource() before and after the change, exactly like this
chapter's own verification method - are confirmed byte-identical.
Neither needed to be opened, read, or reasoned about to make this
change correctly.

VERIFYING THE NEW THRESHOLD BEHAVES CORRECTLY
------------------------------
    total=120 (was >100, now not >150) old vs new: 115 120
    total=160 (still qualifies under new threshold): 155

A total of 120 - which qualified for the bulk discount under the old
threshold (giving 115) - correctly no longer qualifies under the new
one (staying at 120, undiscounted). A total of 160 still qualifies
(155, discount applied), confirming the new threshold works correctly
in both directions, not just the direction that happens to match the
example given.

WHY THIS CONFIRMS THE ISOLATION PROPERTY GENERALIZES
------------------------------
This chapter's own platinum-tier example showed apply_bulk_discount and
apply_express_fee staying untouched while apply_tier_discount changed.
This exercise shows the REVERSE - apply_tier_discount and apply_
express_fee staying untouched while apply_bulk_discount changes. The
isolation isn't a property of the platinum-tier change specifically;
it's a property of the design itself - whichever single function owns
a given piece of logic is the only one that ever needs to change when
that specific logic changes.

WHY THIS WORKS AS AN ANSWER
------------------------------
A genuinely different change (a threshold value, not a new dictionary
entry) is applied to a different function than this chapter's own
example, using the identical source-comparison verification technique,
and the new behavior is checked in both directions (a value that lost
eligibility, a value that kept it) rather than only the one case that
matches the change's own intent.