public void ParseFail(string text)
        {
            var pu1 = PrivateUseSubtags.TryParse(text);

            Assert.That(pu1.HasValue, Is.False);

            Assert.That(PrivateUseSubtags.TryParse(text, out _), Is.False);

            Assert.Throws <FormatException>(() => PrivateUseSubtags.Parse(text));
        }
        public void Equals()
        {
            var tag1 = new PrivateUseSubtags();
            var tag2 = (object)tag1;

            var tag3 = PrivateUseSubtags.Parse("x-aaa");
            var tag4 = (object)PrivateUseSubtags.Parse("x-aaa");

            var tag5 = PrivateUseSubtags.Parse("x-aaa-bbb");

            Assert.IsFalse(tag1.Equals(null));
            Assert.IsTrue(tag1.Equals(tag2));
            Assert.AreEqual(tag1, tag1);
            Assert.AreNotEqual(tag1, tag3);
            Assert.AreEqual(tag3, tag3);
            Assert.AreEqual(tag3, tag4);
            Assert.AreNotEqual(tag4, tag5);
        }
        public void Contains(string?subtagText, string tag, bool expected)
        {
            var pu = subtagText is null ? new PrivateUseSubtags() : PrivateUseSubtags.Parse(subtagText);

            Assert.That(pu.Contains(tag), Is.EqualTo(expected));
        }