https://en.wikipedia.org/wiki/Opaque_set
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Leo Crabbe
The left-sorted patterns connect all four corners with a single black region. - Rhodri Prysor, Oct 16 2024
Adjacent-numbered pages: BP1247 BP1248 BP1249 BP1250 BP1251  *  BP1253 BP1254 BP1255 BP1256 BP1257
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