Thanks! A pencil tool should be straightforward to add. Do people usually use a stylus for these purposes? Someone else mentioned the ads, I can easily embed some mechanism to remove them.
You can keep ads, but the sudoku app I use gives you a few hints for free, first.
Pencil tool is usually a toggle- you default to `pen` but toggle `pencil` mode to fill them out. Look at the top android sudoku app's implementation, it's slick.
What type of hints are most useful from your POV? Are they more direct (i.e. here's a free box in case you are stuck), or more suggestive (e.g. "Pay attention to the naked pair in the third grid")?
With the latter, striking a nice balance between hints which are simple to articulate without esoteric terminology is important.
Really appreciate the pencil mode pointer, I'll check it out.