public void it_sums_empty_collections()
        {
            var expected = new[] { 0, 9 };
            var actual   = Sum.AddTails(
                new int[] {},
                new[] { 3, 4, 5 }
                );

            actual.Should().Equal(expected);
        }
        public void it_sums_tails()
        {
            var expected = new[] { 2, 9 };
            var actual   = Sum.AddTails(
                new[] { 1, 2 },
                new[] { 0, 9 }
                );

            actual.Should().Equal(expected);
        }