Hmmm. Maybe the above comment was poorly worded by me years ago... I forget.

IIRC, as the "walker" when you come to a "+" then you MUST turn. There might be a straight ahead path from the "+" (as seen from the direction you are facing) but you cannot take it.

It is best just to read the rules from the Kata description and examples. Those will be right. Whereas sometimes very old comments may have been explaining how it worked way back when it was still evolving in Beta.

"The part that says + is "only for corners" is not meant to imply the straight ahead path is disallowed" & "If you find yourself at a corner then you must turn" & ambiguousness rule = very confusing for me.
After one hour reading several times all the comments I finally think that a "+" can really "draws" a path in every direction and even the straight ahead direction has to be considered for the ambiguousness rule. Am I right ?

No, it's like this:

• When you start at the top X and get to the middle + there is a choice of 2 paths - go left or straight ahead
• When you start at the bottom X and get to the middle + there is a choice of 2 paths - go left or right
• Since the path is ambiguous in BOTH directions the line is "invalid"

(BTW - The part that says + is "only for corners" is not meant to imply the straight ahead path is disallowed. It is only saying that at least one of the outgoing paths from the + must be a turn. e.g. This is the reason for the 2nd bad-line example. If you think this Kata description is confusing please suggest something better for consideration).

From kata description:

Rules for valid lines:

...
Every line "character" found in the grid must be part of the line. If extras are found then the line is not valid.

I believe the bottom branch of that grid is classified as 'extras'

but if you start with the top one it's very simple because the second '+' must be a turn

Must it?