[Therion] Extended elevation control - Embedded image does not appear in some layouts

[Bruce Mutton]

> If you want more than one variation of the extended elevation, eg. with the leg folded left in one, and right in the other ... you are in for a small nightmare, and I will let Bruce help you out, because he has a way to do it. 
> It is not straightforward, because Therion is really designed to have just one extended elevation.

I've been on a journey.
In 2018 I decided that extend options in-line with the survey data was inadequate and I experimented with moving all extend controls to a separate extended elevation centreline.  The reasons and details of how are in https://therion.speleo.sk/wiki/extend?s[]=extend#extended_elevations 

Almost immediately I started having problems as loops were added to ongoing survey projects.  New loops cause existing extended centrelines and drawings to be broken, requiring careful (often time consuming) extend control edits to resolve.  Until a few weeks ago I thought the problems were due to my choice to move extend control to separate centrelines outside of the primary survey data enumeration.

With my most recent project I decided to go back to leaving extend statements in-line with survey data, for simplicities sake (so I thought).  Then I started adding some loops.  What I have found leads me to suspect that the in-line and separate centreline choice makes no difference to the problems caused by use of ignore and addition of loops to existing extended centreline development.  The 'extended elevation additional loops breaks previous loops' problem is a Therion problem, not 'the way we use Therion' problem.  The 'small (extended elevation) nightmare' is associated with multiple loops, not with multiple extended elevations.

Which leads me to thinking that the ultimate solution will be to add to Therion a new extended elevation control set when it comes to 'extend ignore'.  Rather than saying 'when there is a choice, don't take this path' we should instead say, 'when there is a choice, TAKE this path'.  I feel like this would reduce the uncertainty of implying what we want by specifying only one of the options (at a time) that we don't want.  Better to directly specify the ONE option we DO want and not care about the possibly many options we don't want.  This is especially the case when there are more than three legs meeting at a station. If such a change would make all existing extended elevations incompatible, then having a 'UseNewExtendControl' control in Therion would seem to be a tolerable mitigation.  And Therion continues with two extended elevation control sets.
So yes, it's not straight forward! :-)
Bruce