- ➊ Blacken some cells of the diagram [paper] resp. color the
gray cells of the diagram black or white [interactively] and draw a single
closed into the diagram.
- ➋ The number in a cell indicates how many black cells are
orthogonally adjacent to this cell. There may exist some black cells not
adjacent to a numbered cell.
- ➌ Numbered cells are never painted black. Two black cells
must not be orthogonally adjacent.
- ➍ The sections of the loop run horizontally or vertically
between the centers of orthogonally adjacent cells.
- ➎ The loop must visit all empty white cells exactly once (it
may not cross itself).