I've added another km or so - the go-on series. No real problems except that all the pillars were inside-out. I split it into two scraps instead of one huge one, thereby determining in which half the problem lay, but after much staring at it and checking line types and looking at debug mode and fixing various problems, I still can't see why it's confused.
The problem scrap is goon_s2 and it goes wrong on its own as well as when processed as part of the big plan. There are no therion errors about overlapping outlines, although there is one red metapost entry in the list (what do those mean?) The set of files is at http://trilogy-gw.aleph1.co.uk/~wookey/files/terikan.tgz (no pics). As ever, clues welcome. Wookey -- Aleph One Ltd, Bottisham, CAMBRIDGE, CB5 9BA, UK Tel +44 (0) 1223 811679 work: http://www.aleph1.co.uk/ play: http://www.chaos.org.uk/~wookey/
