How about a Reload checkpoint button?
The button will be at the bottom.
So when you press the checkpoint button it will make a check point of where the simulation is.
and if you mess up your simulation and you can't undo more than once, you click the reload button and it will bring the simulation back to the state of when you pressed the checkpoint button.

cracker64

but firstly this is better than doing that.
Secondly.... well it's just better :P