>> Isn't trial and error a form of deduction anyway?
>
>  The most common aid used in solving sudoku (and thus supported by most
> sudoku software) is to put small marks like "in this 3x3 subgrid a 5 goes
> to either this square or this one" (by writing small 5's at the corner of
> the square) and the like. I don't call that "trial and error", but a form
> of bookkeeping.

But then at some point, you are going to think to yourself - if I put the 5 
in this box, that means a 7 must go in that box (maybe some more steps here) 
nope that doesn't work, therefore the original 5 must go in that other box. 
Isn't that trial and error?

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