int isAsciiDigit(int x) {
    int overflower = (1 << 31) ^ (~0x39);
    int overflowed = (x + overflower) >> 31;
    if (overflowed) {
        printf("Overflowed, so not an AsciiDigit.\n");
    }
    return !overflowed & !((x >> 4) ^ 3);
}