Normal sudoku rules apply. Digits may not repeat along the indicated diagonal. Numbers outside the grid are simultaneously full rank clues and sandwich clues. A '?' can be any number. Full rank Each complete row and column can be read from both directions to form 36 distinct 9-digit numbers. The clues outside the grid give the rank of the 9-digit number read from that position when all the numbers are ranked from smallest (1) to highest (36). Sandwich: A number outside the grid represents the sum of the digits sandwiched between the '1' and '9' in its respective row or column.