Exercise 3: Adding Uppercase Support Without Ten Comparisons — Possible Solution
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THE MINIMUM CHANGE
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Rather than duplicating all five vowel comparisons for uppercase
versions (A/E/I/O/U) as five MORE comparisons, add a single
normalization step at the very TOP of ISVOWEL, before any of the
existing five comparisons run: check whether the input character
falls in the uppercase range ('A' through 'Z'), and if it does,
convert it to its lowercase equivalent before falling through into
the existing, unchanged lowercase-only comparison chain.

WHY THIS WORKS: THE ASCII RELATIONSHIP
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In ASCII, every uppercase letter's code is exactly 32 less than its
lowercase equivalent ('A' = 65, 'a' = 97; 'E' = 69, 'e' = 101; and so
on, consistently for the whole alphabet). So converting any uppercase
letter to lowercase is always the same operation: add 32 to its ASCII
value. This is the same idea this chapter's own ISVOWEL already uses
(comparing ASCII values via subtraction) applied to CONVERSION instead
of comparison.

PSEUDOCODE
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ISVOWEL(R0 = character):
    if R0 is between 'A' (65) and 'Z' (90) inclusive:
        R0 = R0 + 32          ; normalize uppercase down to lowercase
    ; fall through into the existing five lowercase-only comparisons,
    ; completely unchanged from this chapter's own version

Testing "is R0 between 'A' and 'Z'" can itself reuse the same
negate-and-add technique this chapter's ISVOWEL already uses for
equality checks -- one subtraction to confirm R0 - 'A' is not
negative (R0 >= 'A'), and one more to confirm R0 - 'Z' is not
positive (R0 <= 'Z'). Only if both hold does the +32 normalization
run.

WHY THIS IS THE "MINIMUM" CHANGE
------------------------------
The existing five comparisons (against VOWEL_A, VOWEL_E, VOWEL_I,
VOWEL_O, VOWEL_U) don't need to change at all -- they still only ever
need to handle lowercase input, because the new normalization step
guarantees that by the time execution reaches them, ANY vowel letter
(uppercase or lowercase) has already been converted to its lowercase
form. This adds one small block of new logic instead of duplicating
the entire five-comparison chain a second time for uppercase.

WHY THIS WORKS AS AN ANSWER
------------------------------
It identifies the actual minimal-change technique (normalize case
once, up front, rather than doubling every comparison), grounds it in
the real ASCII relationship between uppercase and lowercase letters
(a fixed +32 offset), and explains specifically why the existing five
comparisons can stay completely unmodified as a result.