Clues outside the grid give the sums of the digits that neighbour either the 1 or 9 in the row or column. The solver must determine which clues are for ones, and which are for nines. Where two clues appear, they give the sums for both the 1 and 9, and the order in which the sums appear (left to right in a row, top to bottom in a column). Where one clue apperas it could be for either the 1 or the 9. Some clues are replaced with question marks, which represent unknown numbers of any length. Equality and inequality signs give relationships between some pairs of clues.
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