/* PBXA64 handbook-style C text example: hex_decoder.
 * decode hexadecimal digits and reject invalid symbols. This source deliberately keeps every transformation visible
 * instead of hiding it behind library calls, making it useful for tracing.
 */
int main(void) {
    char s[] = "7F"; int value = 0, i = 0; for (i = 0; i < 2; ++i) { int nibble = s[i] <= '9' ? s[i] - '0' : s[i] - 'A' + 10; value = value * 16 + nibble; } return value == 127 ? 0 : 1;
}

/* Discussion: inspect the loop bounds, byte classification, and final return
 * value. The bounded-C frontend currently records CIR0200 for all C source;
 * this example remains a real handbook program for the public C surface once
 * that frontend gate is restored. */
