Normal sudoku rules apply. All friendly cells in the grid form a single one-cell-wide orthogonally-connected snake, which cannot touch itself, even diagonally (a cell is 'friendly' if it has a value identical to its row-, column- or box-number). Cells connected by a dot are both friendly; if they are the same 'type' of friendly (that is, both match the row-number, both match the column-number, or both match the box-number) the dot is white, and otherwise the dot is grey.