Plan 9 from Bell Labs’s /usr/web/sources/contrib/anothy/src/lib/djb-ape/byte/case_lowers.c
#include "case.h" void case_lowers(s) char *s; { unsigned char x; while (x = *s) { x -= 'A'; if (x <= 'Z' - 'A') *s = x + 'a'; ++s; } }