@Test
public void testSum() {

  List<BigInteger> summands = Arrays.asList(
      BigInteger.ONE,
      BigInteger.ZERO,
      modulus.subtract(BigInteger.TEN).mod(modulus),
      new BigInteger("42").mod(modulus)
  );
  BigInteger expected = BigInteger.ONE.add(BigInteger.ZERO)
      .add(modulus.subtract(BigInteger.TEN).mod(modulus)).add(new BigInteger("42").mod(modulus))
      .mod(modulus);
  assertEquals(expected, MathUtils.sum(summands, modulus));
}