After more consideration, I think I have this backwards.
For the next release, I'll switch it so that 0 clears the cell, and holding 0 clears only invalid marks. And while I'm at it, I think I'll make holding 0 also promote a single pencil mark to a pen mark.
The help file is a good idea, especially since the number of features is rapidly growing. I do want to finish up some other features first, though.
Thanks for the feedback!