How to change settings for just a few readings?

[Olly Betts]

I'm looking at how to convert from .svx format to .th format.  Mostly
working, except I can't see how to handle this cleanly (the situation
modelled here is that the tape is too short, and for one leg is was
measured from a different point - in the example this was distilled
from because 2-3 was a plumb and we needed more tape to tie a crab on):

  *begin test

  *calibrate tape 1
  1 2 10 0 0

  *begin
  *calibrate tape 2
  2 3 10 0 0
  *end
  ; *calibrate 1 is now be back in effect

  3 4 10 0 0

  *end test

The nearest I can get is:

  encoding iso8859-1
  survey test -title "test"
  centreline
  calibrate tape 1
  1 2 10 0 0
  endcentreline
  centreline
  calibrate tape 2
  2 3 10 0 0
  endcentreline
  centreline
  3 4 10 0 0
  endcentreline
  endsurvey

However this doesn't restore the "calibrate tape 1" of course (the
survey length is 27m).  I can't omit the survey name for a survey
command, or pass an empty name.  And I can't nest centreline commands.

The only solution I can see is to track which settings are in effect and
reset them all after the *end.  That's just about OK for automated
conversion if nobody's going to want to edit the result, but it must be
a right headache for anyone who has tried to hand convert a dataset so
I'm wondering if there's some way to express this situation.

Cheers,
    Olly