public async Task AttemptingToAppendANullSequenceToANonNullSequenceThrowsArgumentNullException()
        {
            var source = new CelestialBody[0].ToAsyncEnumerable();

            IEnumerable <CelestialBody> additional = null;

            source.ShouldNotBeNull();

            additional.ShouldBeNull();

            await source.AppendAsync(additional)
            .ConsumeAsync()
            .ShouldThrowAsync <ArgumentNullException>();
        }