After filling in all the numbers I can deduce without guessing, I lightly fill in every remaining box with all the possible numbers. Now to make a guess I pick one number and mark it with a light dot above it. Then continue, using standard methods to find other numbers, marking all these new numbers with dots. Hopefully one can continue on to either a solution or a contradiction. If the latter, you then erase all the dots and remove the originally-guessed number that's turned out to be impossible.
Bill
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