public void can_mark_the_missing_grammars()
        {
            var system = new FixtureModel("something");

            system.AddGrammar(new Sentence {
                key = "Go1", format = "Go One"
            });

            var overrides = new FixtureModel("something");

            overrides.AddGrammar(new Sentence {
                key = "Go2", format = "Go Two"
            });

            var combined = system.ApplyOverrides(overrides).As <FixtureModel>();

            combined.FindGrammar("Go1").IsMissing.ShouldBeFalse();
            combined.FindGrammar("Go2").IsMissing.ShouldBeTrue();
        }