Normal sudoku rules apply. Divide the grid entirely into '15-clumps' - non-overlapping orthogonally connected groups of cells, each of which contains no repeated digits and sums to 15. An arrow in a grid indicates that exactly N 15-clumps can be seen in the direction of the arrow, where N is the digit in the arrow's cell. The arrow's cell is not counted, though that cell's clump may be counted if other cells in it are visible in that direction. Two arrows in the same cell are counted as independent clues.