public void CannotAttachTrailerTwice()
        {
            var trailerHook = new TrailerHook();

            Assert.Throws <DomainRuleViolatedException>(() =>
            {
                trailerHook.AttachTrailer();
                trailerHook.AttachTrailer();
            });
        }
        public void AttachingTrailerResultsInEvent()
        {
            var trailerHook = new TrailerHook();

            var events = trailerHook.AttachTrailer();

            Assert.True(events.Any(x => x is TrailerHookBecameOccupied));
        }
        public void DetachingTrailerResultsInEvent()
        {
            var trailerHook = new TrailerHook();

            trailerHook.AttachTrailer();
            var events = trailerHook.DetachTrailer();

            Assert.True(events.Any(x => x is TrailerHookStoppedBeingOccupied));
        }