Can someone explain in the puzzle below why the following step is called an Empty Rectangle? Also, I do not understand the notation r4c3=4=r4c4. I understand that one of those cells has to be a 4, but the notation is confusing to me.
Here is the step from the Sudoku Solver program:
3. Empty Rectangle in n7 r4c3=4=r4c4
3a. Candidate 4 removed from r4c789 r6c67 r5c6
I understand 3a, my question only involves the ER portion.
Perhaps a little explanation on how to identify where the ER technique exists in a jigsaw puzzle would be helpful, too.
Regards,
George
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