Normal sudoku rules apply. The lavender lines are zipper lines. Each pair of digits equidistant from the centre digit has a sum equal to that centre digit. For instance, 16827 would be a valid fill for a zipper line because 8 = 1 + 7 = 6 + 2. The darker off-green kind-of-unpleasant-looking lines are modular lines. Each set of three adjacent digits along a modular line must contain one of 147, one of 258, and one of 369.