[Therion] Strange scale-dependent Metapost error

[Duncan Collis]

Turns out that this one:

> Removing non-selected maps (if comment out in the master .th file for
> the area maps that aren't selected in the .thconfig file that causes
> the problem, it compiles without errors).

wasn't true - I've managed to shrink the problem down to a data set
consisting of:

custom symbol definitions
survex .3d file
.th2 files for one cave
.th2 file containing a surface grid
stripped-down .thconfig file
a couple of .th files that tie things together.

The whole thing now fits in a <200K .zip file, which I will send to
Martin Budaj  It's quite a large minimal sample, but I'm struggling to
find anything else that I can remove or change without the error
vanishing.

to Martin B: thanks for offering to look into this!

Duncan.