Setting function return type

[SamstaUK]

I want my function to return the type mpz_t, but I'm not sure how?

I've tried:
mpz_t MyFunction(mpz_t A, mpz_t B){}

But it didn't work, here is my code so far, I have bolded the parts of the code which are causing errors and added the errors in the comments:

#include <iostream>
#include <mpir.h>

using namespace std;

???? A(mpz_t m, mpz_t n){
	mpz_t mmo; //used to store the value of m, minus one
	mpz_t nmo; //used to store the value of n, minus one
	mpz_t one; //used to store the value 1
	mpz_init(mmo);
	mpz_init(nmo);
	mpz_init(one);
	mpz_set_ui(one, 1);

	if(mpz_cmp_ui(m, 0) == 0){
		mpz_add_ui(n, n, 1);
		return n; //error: return value type does not match the function type
	}

	if(mpz_cmp_ui(m, 0) > 0 && mpz_cmp_ui(n, 0) == 0){
		mpz_sub_ui(mmo, m, 1);
		return A(mmo, one);
	}

	if(mpz_cmp_ui(m, 0) > 0 && mpz_cmp_ui(n, 0) > 0){
		mpz_sub_ui(mmo, m, 1);
		mpz_sub_ui(nmo, n, 1);
		return A(mmo, A(m, nmo)); 
	}
}

int main(){
	mpz_t param1;
	mpz_init(param1);
	mpz_t param2;
	mpz_init(param2);

	mpz_set_ui(param1, 4);
	mpz_set_ui(param2, 2);

	mpz_out_str(stdout, 10, A(param1, param2)); 
	return 0;
}

Is there a way around this?

Thank you