Another color to mark in or a way to go back?

[Cawingcrow]

It would be really great to have an option to switch to a different color mark (or just a different symbol) partway through so when you are wandering down a path you think is right and it turns out to be a dead-end, you can just go back and take out those marked spaces instead of having to clear everything for just give up. Or else a way to go back sequentially until you get to the point where you started screwing up. :)

[pcuseralso]

Great idea. I often use a math method of working to solution or contradiction to quickly find an answer. It would make things much easier with that feature...

[Laura]

I also think it's a great idea! It would mean fewer times clearing the whole grid and starting over.