I notice that;

debug scrap-names

causes a metapost (?) error with the attached drawing file.  Therion almost recovers but reports exit code cp2 and produces no pdf output.

Admittedly the file is incomplete, a work in progress.  There is a scrap with point stations and no walls, and there are two empty scraps.  I think it is the empty scraps which upset the debug process.

However perhaps it would be better if metapost just reported the existence of the empty scrap and then did not try to label it!

(or maybe something else is going on?)

Bruce