#[derive(Event)]
    struct Boom;

    #[derive(Event)]
    struct BoomTomorrow;

    fn boom(trigger: Trigger<Boom>, mut commands: Commands) {
        println!("boom, despawning entity: {:?}", trigger.entity());
        commands.entity(trigger.entity()).despawn();
    }

    fn boom_tomorrow(trigger: Trigger<Boom>, mut commands: Commands) {
        // boom tomorrow
    }

    #[test]
    fn test_boom() {
        let mut app = App::new();

        let mut id = app.world_mut().spawn(Name::new("a")).observe(boom).id();
        let mut id2 = app.world_mut().spawn(Name::new("b")).id();
        
        app.world_mut().flush();
        println!("flushed");
        
        let mut a = app.world_mut().entity_mut(id);
        a.trigger(Boom);
        a.observe(boom_tomorrow);
        let name = a.get::<Name>().unwrap();
        assert_eq!(name.as_str(), "b");
    }