[Therion] How to use user-defined symbols

[Bill Gee]

Hello everyone -

I have been struggling for the last several days trying to figure out how to 
build and use my own symbols.  I need a symbol for two kinds of formations - 
Stalactiflats and Stalagmaflats.  These are basically stalagmites which are fat 
(several feet diameter) and flat on top like a table (or flat on the bottom).

It appears there are two problems to solve.  The first, which I address in this 
question, is how to get xTherion to use a custom symbol.  The second is to 
figure out how to draw the symbol in Metapost.  For now I am ignoring the 
second problem.  One step at a time!

After spending several hours going through Therion By Example, I was able to 
get a custom symbol to work, but only by replacing an existing symbol.  I used 
the NSS "gradient point" example.

Changing the Metapost code as shown does not produce any error messages.  
However, it also does not produce a symbol in the PDF output.  All I get is a 
red dot.

===============
        code metapost
 
        def p_stalagmaflat_AMER (expr pos,theta,sc,al) =
          U:=(.15u, .4u);
          T:=identity aligned al rotated theta scaled sc shifted pos;
          pickup PenC;
  
          #Left Hand side
          thdraw (-.3u, -.2u) -- (-.7u, .1u);
          thdraw (-.2u, -.1u) -- (-.4u, .6u);
  
          #Centerline
          thdraw (0u, 0u) -- (0u, .9u);
  
          #Right side
          thdraw (.3u, -.2u) -- (.7u, .1u);
          thdraw (.2u, -.1u) -- (.4u, .6u);
  
        enddef;
      
      initsymbol ("p_stalagmaflat_AMER");
      
   endcode
==================

Since I am not replacing an existing symbol, I leave out the "symbol-assign" 
line.

In xTherion I try to use this by inserting a point of type "u" and adding an 
option "-subtype stalagmaflat".

I think this is where I am missing something ...  How to call the custom 
symbol with a "u" point.  Is this the right syntax?

In the worst case I suppose I could set it up to replace a symbol that I won't 
use for this cave - a ladder, for example.  That would probably cause quite a 
bit of confusion down the road, so I would much prefer to avoid it.

I am using Therion 5.3.13 on 64-bit Fedora 20.

Thanks - Bill Gee