[Therion] Breaking a loop at a specific station on extended elevation

[Tarquin Wilton-Jones]

> If extend algorithm follows stations in order 1 - 2 - 3 - 4 - 5 - 6 - 7
> - 8 - 9 and you specify
> extend vertical 3
> extend right 8
> Then stations 3,4,5,6,7 should be placed vertical and 8,9 on the right.
> I have tested it and in my example, it works like this.

In the case of what you said, I would have expected the following legs
to be default (right):

1-2
2-3

the following legs to be vertical:

3-4
4-5
5-6
6-7
7-8

then the following legs to be right:

8-9

Instead, leg 2-3 will be vertical, and leg 7-8 will be right.

At least in my head, "extend vertical 3" feels like it should mean
"everything from station 3 onwards". I think Bruce also expected this,
given what he wrote in the wiki "extend" article:
https://therion.speleo.sk/wiki/extend
"Usually this option will not work as described, unless it is preceded
by one or two extend right <leg> statements."

So perhaps this is either a generic misunderstanding; Therion assumes
that by <station> it means <the leg that goes *to* the station>. We
assume that by <station> it should be <the legs that come *after* this
station>.

I don't know which of those is right. I have also never used this with
Survex, so I don't know if Survex agrees with Therion - Perhaps Wookey
can say?