[Therion] Revisiting Breaking extended elevations on specific stations

[Tarquin Wilton-Jones]

> Well, there is one workaround. But to be honest, I have no idea why :/
> 
>     extend ignore 7 8
>     extend ignore 8 7

Wow!

First thought is; that ... really shouldn't work, and it looks almost
like a bug.

Maybe there is logic though, if you look at those two statements in the
opposite order (which also works, by the way).

Therion proceeds along 1-2-10-9-8-7-11
"Therion, please ignore 8-7"
Therion breaks the survey at 8. Therion now goes:
    10-9-8    8
   /         /
1-2-3-4-5-6-7-11
"Therion, please ignore 7-8"
Therion breaks the survey at 7. It now has a floating 7-8 leg not
connected to anything. So it connects it to the 10-9-8 branch, and ends
up with the desired output.

So maybe this is trustworthy rather than a bug? But it hurts my head to
read it, and it smells like there will be some unpredictable hidden
behaviours in there.

> Something like:
> 
>   extend end 8 7
> 
> ... would be definitely better. Is it OK to implement it like that?

I like that it will never clash with existing stuff. I don't like that
it introduces a new keyword instead of "ignore", that does the same
thing as "ignore". I would personally prefer it as a modifier rather
than a keyword, for consistency:

extend ignore end 8 7
extend ignore 8 7 end

But others may have stronger opinions.