Draw lines along the grid edges, in such a way that the grid is divided into connected regions, all of the size shown in the status line. Also, each square containing a number should have that many of its edges drawn in.
Click on a grid edge to mark it as a division between regions (black), and again to return to marking it as undecided (yellow). Right-click on a grid edge to mark it as definitely not part of the loop (faint grey), and again to mark it as undecided again.
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