prize18-3  Exercise 1: Flip Case with One Bit
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Idea: in ASCII, "A" is 0100 0001 and "a" is 0110 0001. They differ only in bit 5 (value 0x20), so XOR with 0x20 toggles case.
It must only be applied to ASCII letters. XOR on other characters would corrupt them (for example it would turn a digit or a space into something else),
and an accented letter such as e-acute is not in ASCII, so its code point is left alone.

ex1.py:

  def flip_case(s):
      return "".join(chr(ord(c) ^ 0x20) if c.isascii() and c.isalpha() else c for c in s)

  print(flip_case("Hello, World 42!"))
  ok = all(flip_case(chr(c)) == chr(c).swapcase() for c in range(128))
  print("matches str.swapcase on all of ASCII:", ok)
  print(flip_case("caf\u00e9"))

Real output (Python 3.14):

  hELLO, wORLD 42!
  matches str.swapcase on all of ASCII: True
  CAFé

The e-acute is untouched because the test c.isascii() is false for it. Its case rule is not "one bit": e-acute is U+00E9 and its capital is U+00C9,
which happens to differ by 0x20 as well here, but that is a Latin-1 coincidence, not a general rule, and other scripts do not follow it.