internal void Add_Returns_SameInstance(SimpleHashSet <string> target, string added)
        {
            bool success;
            var  res = target.Add(added, out success);

            res.Should().BeSameAs(target);
        }
        internal void Add_UpdateDicionaryAsExpected(SimpleHashSet <string> target, string added)
        {
            bool success;
            var  expected = new HashSet <string>(target);
            bool result   = expected.Add(added);

            var res = target.Add(added, out success);

            res.Should().BeEquivalentTo(expected);
            result.Should().Be(success);
        }