Exercise 2: Adding __sub__ to Money — Possible Solution
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THE NEW METHOD
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    def __sub__(self, other):
        if self.currency != other.currency:
            raise ValueError(f'Cannot subtract {self.currency} and {other.currency}')
        return Money(self.amount - other.amount, self.currency)

Follows this chapter's own __add__ exactly - the identical currency
check, the only difference being subtraction instead of addition and
an error message worded for subtraction specifically.

VERIFYING SAME-CURRENCY SUBTRACTION WORKS CORRECTLY
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    same-currency subtraction: 15.0 USD

Money(25.0, 'USD') - Money(10.0, 'USD') correctly returns 15.0 USD.

VERIFYING CROSS-CURRENCY SUBTRACTION IS CORRECTLY REJECTED
------------------------------
    cross-currency subtraction correctly rejected: Cannot subtract USD and EUR

Money(25.0, 'USD') - Money(10.0, 'EUR') correctly raises ValueError,
exactly mirroring this chapter's own __add__ finding for the identical
kind of mismatched-currency mistake.

WHY THIS CONFIRMS THE PATTERN GENERALIZES BEYOND ADDITION
------------------------------
This chapter's own Money example only verified the invariant (never
combine two different currencies) for addition specifically. This
exercise confirms the SAME invariant, enforced the SAME way, correctly
applies to a second operation without needing any new validation logic
invented from scratch - the currency check itself is the reusable part;
only the arithmetic operation performed after the check changes.

WHY THIS WORKS AS AN ANSWER
------------------------------
The new method mirrors this chapter's own __add__ exactly in structure,
and both the correct same-currency case and the correctly-rejected
cross-currency case are verified directly, confirming the Money type's
own core guarantee (never silently combine two currencies) holds for a
second operation, not just the one originally demonstrated.