Well it works as it should, the algorithm doesn't checks to see if the puzzle has an unique solution (that needs even more recursion than what it has right now) but even then all puzzles should be solvable (even if they aren't a true Sudoku).
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I suppose that's true, they are solvable, but only by just trying something, you can't deduce the solution because you can get to a point where there's 2 possibilities and both are valid. That's why generally all sudoku have a unique solution. Still, I understand that that would be really hard to do and a big pain with such a limited engine. Good work nonetheless!