Normal sudoku rules apply. (ie Place the digits 1-9, once each, in every row, column and 3x3 box.) Cells separated by a single knight's move (in chess) cannot contain the same digit. In a 3x3 box every horizontal threesome contains one digit from {1,2,3}, one from {4,5,6} and one from {7,8,9}. A positive diagonal is any diagonal going from South-West to North-East. When normal sudoku rules allow neighbouring digits on a positive diagonal to be equal they MUST be equal. eg r2c4, r3c3 and r4c2 must contain the same digit; r6c2 and r7c1 must contain the same digit; etc.