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Digits outside the grid are either X-sum values, skyscraper values, or sandwich values.

The outermost grid digit of a row or column fully determines if the outside value is a X-sum, skyscraper, or sandwich. For example, if R1C5 is a 5 and column 5’s outervalue of 15 is determined to be an X-sum clue, then every outermost grid digit 5 is being acted on by an X-sum clue.

X-Sums: Clues outside the grid give the sum of the first X numbers placed in the corresponding row or column, where X is digit in the cell next to the X-sum clue.

Skyscraper: Grid digits represent skyscrapers with heights equal to their values. Taller buildings hide shorter ones behind them. Clues outside the grid show how many skyscrapers are visible from that side in the indicated row or column.

Sandwich: Digits between 1 and 9 in the row or column must sum to the clue outside the grid.

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