Normal sudoku rules apply. In addition, some cells must be coloured grey. Grey cells may not be orthogonally adjacent and the non-gray cells have to form an orthogonally-connected set. All cells (grey and non-grey) must, as usual, contain a digit. A number in the top left corner of a cell indicates the sum of the digits that are “seen” from this cell looking to the north, south, east and west. The cell with the clue counts once in this sum. Grey cells block the visibility. A cell with a clue may not be coloured grey. Within the field of vision of a clue, digits may repeat if the sudoku rules allow it.