// Schedules can be added to other schedules
app.add_schedules(Update, Child);
app.add_schedules(Child, (GrandchildOne, GrandchildTwo));

// Add systems to schedules directly, no appending `.in_set(...)` to everything!
app.add_systems(Update, update_system);
app.add_systems(Child, child_system);
app.add_systems(GrandchildOne, grandchild_system_one);
app.add_systems(GrandchildTwo, grandchild_system_two);

// Sets configured manually, Update must be prepended to everything
app.configure_sets(Update, Child);
app.configure_sets(Update, (GrandchildOne, GrandchildTwo).chain().in_set(Child));

// Adding systems to sets requires `.in_set(...)`
app.add_systems(Update, update_system);
app.add_systems(Update, child_system.in_set(Child));
app.add_systems(Update, grandchild_system_one.in_set(GrandchildOne));
app.add_systems(Update, grandchild_system_two.in_set(GrandchildTwo));

// Initialize the state
app.init_schedule_state::<GameState>();

// Add it to our Update loop
app.add_state_to_schedule::<GameState>(Update);

// Add systems to the state
app.add_systems(GameState::Menu, menu_system);
app.add_systems(GameState::Playing, playing_system);

// Initialize the state, pretty much the same
app.init_state::<GameState>();

// Add systems to our update loop, but we need to manually check on every frame if the state is active
app.add_systems(Update, menu_system.run_if(in_state(GameState::Menu)));
app.add_systems(Update, playing_system.run_if(in_state(GameState::Playing)));