// Nested to "Melee" group systems.
class MeleeSystem1 : IEcsRunSystem {
    public void Run (EcsSystems systems) { }
}
class MeleeSystem2 : IEcsRunSystem {
    public void Run (EcsSystems systems) { }
}

class MeleeGroupEnableSystem : IEcsRunSystem {
    public void Run (EcsSystems systems) {
        // We can enable "Melee" group with special event.
        var world = systems.GetWorld ();
        var entity = world.NewEntity ();
        ref var evt = ref world.GetPool<EcsGroupSystemState> ().Add (entity);
        evt.Name = "Melee";
        evt.State = true;
    }
}
...
// startup code.
var systems = new EcsSystems (new EcsWorld ());
systems
    // Adds disabled group "Melee" with 2 nested systems,
    // group-events will be stored in default world.
    .AddGroup ("Melee", false, null,
        new MeleeSystem1 (),
        new MeleeSystem2 ())
    // Other systems.
    .Add (new MeleeGroupEnableSystem ())
    .Init ();

var systems = new EcsSystems (new EcsWorld ());
systems
    .Add (new System1 ())
    .Add (new System2 ())
    // All C1 components will be removed here.
    .DelHere<C1> ()
    .Add (new System3 ())
    // All C2 components will be removed here.
    .DelHere<C2> ()
    .Add (new System4 ())
    .Init ();