@dr who what algoirthm is used for your sroce? IMHO 27 for only cross hatching is very high
Mr. O, the algorithm is explained on my web page (see my profile for the link). My algorithm agrees with your analysis that only cross hatching is required. However, I distinguish cross hatching a row or column from cross hatching a block. I believe cross hatching rows or columns is harder than cross hatching a block. So there were probably several rows or columns to be cross hatched and probably a lot of dependencies (where one cross hatch cannot be seen until several others have been found).