[Therion] Problem using extend command

[John Pybus]

Bruce Mutton wrote:
> And hi again
>  As I have mentioned I have very little experience with extended elevations,
> but taking your example as an opportunity to learn...
> 
> The following excerpt from the Therion book suggests you may have made an
> error in input.  Perhaps if you specify only one station it will work as you
> desire?
> 
> "normal/reverse . extend given and following stations to the same/reverse
> direction as previous station. If two stations are given, direction is
> applied only to given shot.

Ah, that was not my understanding of how it works.  My reading of that 
passage is saying that the directive applies only to the given shot at 
the 1st station (specified by the 2nd station) not that it then stops 
applying to further stations in that direction.  It is useful at 
junctions where the one station version isn't specific enough.

i.e.   1--2--3
           |
           4--5

extend reverse 2
would give:

1,3==2
      |
   5--4

(i.e. both 2->3 and 2->4 get affected)
and
extend reverse 2  4
would give:

  1--2--3
     |
  5--4

(i.e. just 2->4 direction gets affected)

That is how the equivalent function works in survex espec files, so how 
I expected the functionality in therion to work.

I can't really see any use for the meaning Bruce describes, but the 
meaning I drew is frequently useful at junctions in the cave.  I also 
think it's a bad idea for therion to have commands with the same syntax 
as survex but gratuitously different syntax.

Perhaps it could be changed to match?

[disclaimer: I did write the survex support for extend specifications, 
though that was many years ago]

John