// Embed a "fake" exit code in the middle to check that we don't get caught on it.
	pre := "abc" + exitStr + "123def"
	text := pre + exitStr + `1`
	for i := 0; i < len(text); i++ {
		_, err := f.Write([]byte{text[i]})
		if err != nil {
			t.Fatal(err)
		}
	}

	// The "pre" output should all have been flushed already.
	if want, got := pre, out.String(); want != got {

}

// ProbablyPrime performs n Miller-Rabin tests to check whether z is prime.
// If it returns true, z is prime with probability 1 - 1/4^n.
// If it returns false, z is not prime.
func (z *Int) ProbablyPrime(n int) bool {
	z.doinit()
	return int(C.mpz_probab_prime_p(&z.i[0], C.int(n))) > 0