return "nil"
	}
	z.doinit()
	p := C.mpz_get_str(nil, 10, &z.i[0])
	s := C.GoString(p)
	C.free(unsafe.Pointer(p))
	return s
}

func (z *Int) destroy() {
	if z.init {
		C.mpz_clear(&z.i[0])
	}
	z.init = false
}

/*
 * arithmetic
 */

// Add sets z = x + y and returns z.
func (z *Int) Add(x, y *Int) *Int {
	x.doinit()
	y.doinit()
	z.doinit()