#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <assert.h>

#define ENABLE_DEBUGGING_CODE
// Define a “symbol” ENABLE_DEBUGGING_CODE

// #ifdef happens at COMPILE TIME, not when your program is running.

#ifdef ENABLE_DEBUGGING_CODE
    // ENABLE
#   define dbg_log_int(n) (printf("# %s == %d\n", (#n), (n)))
    // Reminder:  #n means the text of the expression passed to the function-like macro.
#else
    // DISABLE
#   define dbg_log_int(n)
    // Invocations to the function-like macro disappear.
    // ∙ dbg_log_int(divisor);  ⇒   ;
#endif

bool is_prime(int n) {
    bool is_n_prime = (n % 2) != 0;  // Assume n is prime unless we find that it is divisible
                                     // by something other than 1 or itself.

    // Try all ODD numbers from 3 to n/2
    if(is_n_prime) {
        for(int divisor = 3; divisor < n / 2; divisor += 2) {
            dbg_log_int(divisor);
            assert(divisor % 2 == 1);  // "I claim that divisor must be odd."
            if(n % divisor == 0) {
                is_n_prime = false;
                break;
            }
        }
    }

    assert(n == 2 || n % 2 == 1 || is_n_prime == false);  // "I claim that n cannot be prime and even unless n is 2."
    assert(n == 2 || n % 2 == 1 || ! is_n_prime);         // "I claim that n cannot be prime and even unless n is 2."

    return is_n_prime;
}

int main(int argc, char* argv[]) {
    int n = 13;
    if( is_prime(n) ) {
        printf("%d is prime", n);
    }
    return EXIT_SUCCESS;
}
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