I have just started learning Therion, and after drawing up my first survey I got some feedback. The main point was to use local symbols. So I set about trying to convert my survey to use the NSS standard. Reading the Therion Book it seemed like it wasn't supported, as it is not mentioned on page 51 (along with some other standards that are actually supported). 

Searching some more I found that it was listed in https://therion.speleo.sk/wiki/_media/therion_symbols_20130407_.pdf along with several other <symbol-set>s not mentioned in the Therion Book. I wasn't sure if if these should be added, and thought i'd send this to make ppl aware.

Regards,

Felix