Normal sudoku rules apply. ie place the digits 1-4, once each, in every row, column and 2x2 box. Some of the grid is covered by fog. Entering a correct digit will clear the fog from the surrounding cells. There is no need to guess. Digits along a grey line form a palindrome ie the digits along the line must read the same forwards and backwards. Palindromes move orthogonally and NOT diagonally. A digit on a palindrome shows how many times that digit appears on that palindrome.