No Single Edge
An untouched dot with only one possible edge cannot use it, because that would leave exactly one line at the dot.
How it works
- 1Three directions at the middle dot are already crossed out, and no line reaches it.
- 2Drawing the one remaining edge would leave the dot with exactly one line — a dead end, not part of a loop.
- 3Cross out the last edge too; this dot stays untouched.
- Edge ruled out
- Cells that establish the pattern