public void OnePropertyValueShouldBeAppended(PropertyValueCollection collection)
        {
            // Fixture setup
            collection.SetValue(DirectoryProperty.Member, DN.Parse(john));

            // Exercise system
            collection.AppendValue(DirectoryProperty.Member, DN.Parse(paul));

            // Verify outcome
            collection.GetValues <DN>(DirectoryProperty.Member)
            .Should().BeEquivalentTo(DN.Parse(john), DN.Parse(paul));
        }