[from Nikoli website]
- Lines must pass through the centers of cells horizontally or vertically and never cross, branch off, or go through the same cells twice.
- Cells where the line is drawn must not be black.
- Black cells must not touch each other horizontally or vertically.
- Lines don't go through numbered cells. Numbered cells cannot be black cells.
- Numbers represent how many black cells are at the direction of the arrow.