[Therion] Hiding centreline within a scrap, while showing it for surveys

Hi folks, I know this must be a common desire, but I can't seem to find it on the wiki or Therion book. map foo bar@baz #a map containing scraps qux #a survey endmap This will show the scraps within bar@baz, and the centreline for qux. All good. I want to hide the centreline within bar@baz, since it has passage walls, and no need to show a centreline. But I need to continue showing the centreline for qux, since it has no passage wall data, only the centreline. I am sure I have seen this on John's surveys, but I can't seem to work out how to do it (all I can see is a mail from Stacho in 2007 asking for clarification whether someone actually wanted that functionality). Is there a way?