Normal sudoku rules apply. Draw a one-cell-wide loop of orthogonally connected cells, which MAY touch itself orthogonally or diagonally, and must visit each box at least once. Two different cells on the loop are called partners if it takes the same amount of steps along the loop to reach each other in both directions. The difference in value between two partner cells is different for each pair of partner cells. Digits in circles must appear in the surrounding four cells at least once. The grey perimeter can be used for making notes.
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