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Python

ASCII_UPPERCASE_START = ord("A")
ASCII_UPPERCASE_END = ord("Z")
ASCII_CASE_OFFSET = ord("a") - ord("A")
def lower(word: str) -> str:
"""
Convert ASCII uppercase letters in a string to lowercase.
>>> lower("wow")
'wow'
>>> lower("HellZo")
'hellzo'
>>> lower("WHAT")
'what'
>>> lower("wh[]32")
'wh[]32'
>>> lower("whAT")
'what'
"""
result = []
for char in word:
code = ord(char)
if ASCII_UPPERCASE_START <= code <= ASCII_UPPERCASE_END:
char = chr(code + ASCII_CASE_OFFSET)
result.append(char)
return "".join(result)
if __name__ == "__main__":
from doctest import testmod
testmod()