how to remove/stop system?

[SET001]

I want to implement Hud system that would add, update and remove all the UI elements. The thing is, I want this to work only on few game states. So I need to be able to start system in on_start and to stop them in on_stop.
I suppose, I can add system using Builder.add_system But how can I remove system?

[simast]

You can build multiple Schedule's and then run one of them based on your game state. I don't think you can modify existing schedule.

[SET001]

how do I select which schedule to run inside game state?