Shade some cells such that shaded cells form four-cell “shards” that may touch on a vertex, but not an edge.
If a vertex has a black circle, then the eight cells in the 2x2 square surrounding that black circle must contain at least one shard with none of its pieces outside the 2x2 square. If a vertex has a white circle, then the eight cells in the 2x2 square surrounding that vertex must be rotationally symmetric about that point and at least one cell must be shaded.
All unshaded cells must be connected edge to edge.
(Rules and example from WPC 2017 IB)
History of the puzzle
Appearances in the past WPCs
- WPC 2017/Round 18 by Rohan Rao