Is there already a space in the wiki with shared symbols?
https://therion.speleo.sk/wiki/metapost#general_symbol_examples
BTW Therion already automatically substitutes symbols that are missing from your chosen symbol set, with those from another.
How does it work exactly? What is the list of symbol sets searched by Therion? In which order are they searched? I couldn't find references to this in Therion Book (in another quick search). It is in the metapost code I think, not hard to find, but in a rush just now.
You can set in your layout different substitutions if you do not like the defaults. How?
For example… #CAVE APPEARANCE #--------------- symbol-set NZSS #if left as default, uses a variety of the following; # UIS, BCRA, NSS, ASF, CCNP, SKBB #Adopt passage slope 'fat triangle arrows' in favour of default skinny arrows symbol-assign line gradient BCRA symbol-assign point gradient BCRA # symbol-assign area blocks AUT # this one is too dense symbol-assign line wall:blocks AUT symbol-assign point blocks AUT symbol-assign area debris AUT symbol-assign line wall:debris AUT symbol-assign point debris AUT # symbol-assign area sand AUT symbol-assign line wall:sand AUT symbol-assign point sand AUT Bruce