
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. 


Assume the top edge of "1" is marked with "x". 


In this case, the line can only go downward.
But then there will be two lines around "1" and it conflicts with the rule. 

Therefore this assumption is wrong, and the top edge of the "1" will be marked with "x". 

Since you can't draw line on the left edge, obviously the loop cannot go through the top left vertex. 
Rule of corner "1"  
