[Therion] More on showing only some cross-sections

[Bill Gee]

A few weeks ago I asked about the possibility of creating a map where 
only some of the cross-sections are shown.  It is easy to include or 
leave out ALL cross-sections, but I would like a way to include only a 
dozen or so.

No one had any usable answers.  There was one suggestion about using 
macros, but not enough detail for me to figure out how to investigate it.

It occurs to me that perhaps some of the cross-sections could be put in 
a custom group or context.  So my first questions:

Is it possible to create a group or a context?  Can I add more than one 
group or context to a symbol such as a cross-section?

Next is a related question.  The owner of the cave I am working on is 
considering creation of a publication of photos they can sell in their 
gift shop.  I think a very useful addition to such a book would be a map 
showing where the photos were taken.

I can, of course, add labels anywhere on the map.  The problem is that I 
want to be able to make maps which do NOT include the photo location 
symbols.

So my next set of questions:

Is there a symbol that might be used to indicate location and direction 
of a photo?  Is there a way to group those symbols so they can be left 
out with something like "symbol-hide group photos"?

This might all be more easily done with custom attributes.  The Therion 
Book has only a few short lines about them.  Is there more documentation 
or perhaps some sample code as to how they might be used?

And my last question is more general.  Is there a list of the predefined 
contexts and groups?

Thanks!
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Bill Gee