[from nikoli website]
- Make a single loop. Lines must pass through the centers of cells horizontally or vertically and never cross, branch off, or go through the same cells twice.
- Lines must pass through all cells containing black and white circles.
- Lines passing through a white circle cell must go straight through the cell, then make a right-angled turn in the very next cell (on at least one side of the white circle cell).
- Lines passing through a black circle cell must make a right angled turn immediately, in the black circle cell, then go straight for the next two cells.