I'm totally confused about the use of extend. I understand right and left reverse but the hide and ignore are puzzling me.

I have a cave which has three entrances with their passages all converging at a common point. I want to show an extended elevation from only one of the entrances and ignore or hide the other two passages.

I've created a simple example of what I want to do.

This is what I get without any modifier. Exactly what I expected.I want to hide the passage at the top (stations 1 to 2 and 2 to 5).

centreline
  extend left
  0 1 10 90 0
  1 2 10 45 45
  2 5 5 90 0
  1 3 10 90 0
  3 4 10 100 0
endcentreline

test 1


The manual says 

extend <spec> [<station> [<station>]] ◃ control how the centerline is extended. 

ignore ◃ ignore specified station (shot), continue extended elevation with other station (shot) if possible

hide ◃ do not show specified station (shot) in extended elevation 


Presuming that (shot) would refer to the two stations 1 to 2 I tried

centreline
  extend left
  0 1 10 90 0
  extend [hide 1 2]
  1 2 10 45 45
  2 5 5 90 0
  1 3 10 90 0
  3 4 10 100 0
endcentreline

but got the error therion: error -- test.th [5] -- unknown extend flag -- hide 1 2

I then read the manual again and tried

…
  extend hide 1 [2]
...

This time I got  

test 2

Not what I expected. The two lines at the top are shots  5 -2 and 1 - 2. I understand why 5-2 might still be there but not 1-2 and why is there no vertical element to 1-2?

I tried again using ignore instead of hide and got the same as test 1.

I then tried

…
  extend hide 1
...

with this result

test 3

Changing hide to ignore gives the following result.
…
extend ignore 1
…

test 4

So, I have two questions.

1. How do I hide or ignore the side passage

2. What is the logic behind the results I get.

Dave Clucas
daveclucas.com
mycaves.org.
daveclucas@icloud.com