+ assert_eq!(&b2 + &b1, BigInt::from_vec(vec![1 << 32, 1]));
+ assert_eq!(&b2 + &b3, BigInt::from_vec(vec![0, 1, 1]));
+ assert_eq!(&b2 + &b3 + &b4 + &b4, BigInt::from_vec(vec![0, 2, 1]));
+ assert_eq!(&b2 + &b4 + &b3 + &b4, BigInt::from_vec(vec![0, 2, 1]));
+ assert_eq!(&b4 + &b2 + &b3 + &b4, BigInt::from_vec(vec![0, 2, 1]));