Also, when the outside two edges of a vertex around "1" are marked with "x" like the right figure, this is considered as corner "1", too.
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The left figure is an examples of corner "1".
Also, when the outside two edges of a vertex around "1" are marked with "x" like the right figure, this is considered as corner "1", too. |
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Assume the top edge of "1" is marked with "x". |
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In this case, the line can only go downward.
But then there will be two lines around "1" and it conflicts with the rule. |
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Therefore this assumption is wrong, and the top edge of the "1" will be marked with "x". |
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Since you can't draw line on the left edge, obviously the loop cannot go through the top left vertex. |
| Rule of corner "1" | |
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