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

int add(int, int);  // Function DECLARATION (aka "forward declaration")

int add(int a, int b) {  // Function DEFINITION
    return a + b;
}

int subtract(int a, int b) {
    return a - b;
}

int operate(int (*operation_fn)(int,int),
            int lhs,
            int rhs) {
    return operation_fn(lhs, rhs);
}

int main(int argc, char* argv[]) {
    int a = 5;
    int b = 6;

    int a_plus_b  = operate(add,      a, b);
    printf("%d + %d = %d\n", a, b, a_plus_b);

    int a_minus_b = operate(subtract, a, b);
    printf("%d - %d = %d\n", a, b, a_minus_b);
    
    return EXIT_SUCCESS;
}
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