Normal sudoku rules apply. A cell is "friendly" if it contains its row, column, or 3x3 box number. [Boxes are numbered in normal reading order ie top left (box 1) through to bottom right (box 9).) All friendly cells are column and row indexers, and they must index at least one cell other than themselves. Column indexing: friendly digits in column X indicate the column in which the digit X appears in that row. Row indexing: Friendly digits in row X indicate the row in which the digit X appears in that column. Friendly indexer example: if r3c6 is a 2, then r3c2 is a 6 and r2c6 is a 3. Digits may not repeat along the indicated diagonal.