Key to solution for Slither Link

General rules

In this example, a line comes to a corner of "2".

	In this example, a line comes to a corner of "2".

These are all cases to draw lines around "2":

Then the common part of above four cases makes a new rule:

Rule of drawing a line to a corner of "2"

To word this, "a line comming to a corner of 2 always goes out from a diagonal vertex."

This may not seem so useful but actually it is, expecially when one of the exit edges is marked with "x".

Of course this rule is applicable when the "2" is on the outmost edge of a puzzle. Because one of exit edges will be blocked.