If you want more than one simulation, you could use the invalidation promise: https://observablehq.com/@observablehq/invalidation Then you would add this to each cell that creates a simulation: invalidation.then(() => sim.stop()); (Another option would be for the AA.Simulation constructor to take an invalidation promise and have it call stop automatically when the promise resolves.)

Great, thanks - I didn't know about the invalidation promise. I think it will be cleanest to use invalidation.then(() => sim.stop()); in every cell that creates a simulation (even if just one simulation in the notebook) and not bother with .single at all in Observable.