// to FFI, just a mismatch between Go's convention of
// making zero values useful and gmp's decision not to.
func (z *Int) doinit() {
	if z.init {
		return
	}
	z.init = true
	C.mpz_init(&z.i[0])
}

// Bytes returns z's representation as a big-endian byte array.
func (z *Int) Bytes() []byte {
	b := make([]byte, (z.Len()+7)/8)
	n := C.size_t(len(b))