public void BeClearable()
        {
            var set = new ImmutableSet64().Add(1);

            set = set.Clear();
            set.Contains(1).Should().BeFalse();
            set.IsEmpty.Should().BeTrue();
        }
        public void RemoveItems()
        {
            var set     = new ImmutableSet64().Add(1, 2, 3);
            var removed = set.Remove(2);

            set.Contains(2).Should().BeTrue();
            removed.Contains(2).Should().BeFalse();
        }
        public void BeImmutable()
        {
            var seta = new ImmutableSet64().Add(3, 12);
            var setb = seta.Add(10);

            seta.Contains(10).Should().BeFalse();
            setb.Contains(10).Should().BeTrue();
            setb.Contains(3).Should().BeTrue();
        }
        public void ContainItems()
        {
            var set = new ImmutableSet64().Add(1, 3, 10);

            for (int i = 0; i < 64; i++)
            {
                set.Contains(i).Should().Be(i == 1 | i == 3 | i == 10);
            }
        }