Fobidoshi 禁四

The board starts with stones scattered across it. Place more stones in the empty cells until every stone — the ones given and the ones you place — belongs to one orthogonally connected group. No row or column may ever hold four stones in a row, so the route between two stones often has to go round rather than straight. Black cells take no stone and cut a line in two.

Three modes: Classic; Lattice, which additionally forbids any solid two-by-two square of stones, so the field has to stay open as it connects; and Sentinel, where numbered cells each count the stones around them.

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Fobidoshi 禁四 — Forbidden Four