The code in the post is based on depth-first search, which basically is a trial and error approach. A well implemented and effective one thanks to constraint pr

The code in the post is based on depth-first search, which basically is a trial and error approach. A well implemented and effective one thanks to constraint propagation, but it is still trial and error. And there are cases, in which backtracking (trial and error) is necessary - at least one of them is documented in the post (the puzzle "hard1" that took 188 seconds).