Notice George Sakkis's comment to the post, the maximize function doesn't work well without the additional unexplained heuristic: Why did you need to change

Notice George Sakkis's comment to the post, the maximize function doesn't work well without the additional unexplained heuristic: Why did you need to change "(assert (and (>= a 1) (<= a 9)))" with "(assert (= a 9))"? Turns out that the former doesn’t find the correct solution, instead it yields 137*803=110011. Whatever the reason I think it’s worth mentioning in the article.