There are four pencil colors available for your use. You may find them
useful in marking your place in the history. For example, you may think
a 7 is in a given cell, but you're not absolutely sure. Mark it in a new color,
say green,
and continue in green. If the choice doesn't work out, you can use undo
to remove all green entries. You end up back at the place where
you made your incorrect choice.