Normal sudoku rules apply. The cells with a clue in the top left corner represent one end of a snake. Each clue belongs to a different snake. The value of the clue gives the sum of all the digits that lie on a snake. These snakes behave as region sum lines. As a snake passes through different 3 x 3 boxes, digits on the snake have an equal sum N within each box it passes through. If the snake passes through the same box more than once, then each individual segment within that box sums to N separately. The snake must pass through at least two 3 x 3 boxes. If '?' is given as a clue, then its value has to be deduced. The snakes can't touch themselves orthogonally or diagonally. Two snakes can't share a cell together.