“…After the publication of this book, a lot of Nikoli's pencil puzzles were shown to be NP-complete. Recently, Double Choco [3], Five Cells and Tilepaint [5], Sukoro [6], Tatamibari [2] Yajisan-Kazusan and Stained Glass [7] were shown to be NP-complete. for each 𝑐 𝑗 ∈ 𝐶 and (ii) the graph 𝐺 = (𝑉, 𝐸), defined by 𝑉 = 𝑈 ∪ 𝐶 and 𝐸 = { (𝑥 𝑖 , 𝑐 𝑗 ) | 𝑥 𝑖 ∈ 𝑐 𝑗 ∈ 𝐶 }, is planar.…”