[Therion] Big room multiple scraps - How to join

[Bill Gee]

Hello everyone - I am having a problem getting Therion to understand how 
to join several scraps to make a large room.  The working map is at

https://campercaver.net/MiscFiles/StarkCavernsNoCenter.pdf

Take a look at the east part of the map in an area called Grand Canyon. 
This is really one large room, though we did not realize that when we 
started sketching several years ago.  Now I have at least three 
different pieces of sketch to try and fit together.

As you can see, there is a large triangle in the center which Therion 
thinks is not cave.  There are also some part which are outside the 
cave, but are considered by Therion to be inside.

All of the joins in here are using line-to-line.  I named the lines, 
then created "join" statements specifying the two lines and which points 
on those lines to join.  Therion is happy with that, and the wall that 
it draws is a fair depiction of the area.  The problem is that much of 
the interior is getting lost.

I have not tried to use plain joins of the scraps - joins which do not 
specify the lines.  I suspect that would not change much.

I have already drawn some of this area three times.  It is 
extraordinarily complicated due to multiple levels, especially along the 
east side.  We thought for a long time that the east side was separate 
passages, but it is really just a complicated pile of big breakdown blocks.

I really REALLY do not want to go back and make one huge sketch of the 
entire room.  That would take several days.

What can I do to convince Therion where the cave really is?  Can I put 
in some invisible walls (subtype hidden)?

Thanks!

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Bill Gee