Normal sudoku rules apply. Digits along a grey thermometer increase from the bulb end. Thermometers contain exactly 4 digits, only move horizontally and vertically (never diagonally) and don't branch or overlap. Thermometers are partially invisible (even after the fog is cleared). The grid is covered in fog. Placing a correct digit in a cell lifts fog from that cell and possibly other cells. Every 2x2 area in the grid contains at least one digit from each of the sets (123), (456), and (789).
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