
The left figure is an example of corner "3".
Also, when the outside two edges of a vertex around "3" are marked with "x" like the right figure, this is considered as corner "3", too. 


Assume the top edge of "3" is marked with "x". 


Then the rest of the edges around that "3" will have lines.
But, in this case the line on the left edge cannot go anywhere. This doesn't satisfy the rule. 

Therefore the assumption is wrong, and the top edge must have a line. 

Since two edges are already marked with "x", the line must go down on the left edge of the "3". 
Rule of corner "3"  
