Sudoku. Nine by nine.

Select one or more cells (drag across them, or shift-click), then tap a number. Notes, Blue and Strike are pencil layers; Auto-pencil keeps candidates up to date as you go. Hint fills the selected cell — with nothing selected, it names a solving technique to try next. Keys: 1–9 place · 0/⌫ erase · N notes · H hint · ⌘/Ctrl-Z undo · P pause.