In addition to standard Sudoku rules: Each cell must have its successor at either its southeast or southwest corner (wrapping around the left/right and top/bottom edges of the grid). The successor of 9 is 1.
For example, if there's a 1 at r9c1, then there must be a 2 at either r1c2 or r1c9.
Links: F-puzzles, Sudoku Exchange
Solution code: Row 7 followed by column 4 (18 digits)
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