Normal sudoku rules apply. MODULAR ARROWS: A digit on an arrow counts how many cells in the indicated direction have the same remainder mod-3 as itself, including itself. (Eg a 4 on an arrow would indicate that exactly four digits (including the 4 itself) in that direction were selected from the digits 1, 4 & 7 (because each of 1,4 & 7 have the same remainder (ie 1) when divided by 3).