[Therion] Symbol for fixed ladder

[Benedikt Hallinger]

Hello,
i get gray hair with metapost while i want to make a line symbol for 
ladders, so i can nicely draw them in my maps as the cave have alot of them.

The basic idea i had was, t define a symbol of one "rung" segment (rung and 
a little of either sides, like the letter "H") as a symbol picture. Then i 
wanted to apply this image seamlessly over the length of the given path 
(straight lines)
What i have for now is the following, however it does not show at all (still 
shows a red line with symbol set AUT in effect), so i have no clue how to 
even debug this.

I tried to start with the example from thbook salted a little with peeks to 
the internal metapost codes.

Can someone assist please?

Thank you very much!


# Symbol for fixed ladder
     def l_fixedladder_SKBB (expr P) =
       T:=identity;
       cas := 0;
       dlzka := arclength P;
       mojkrok:=adjust_step(dlzka, 1.0u);

       pickup PenC;
       sideL := (-.20u,0)--(-.20u,0.20u);
       sideR := (.20u,0)--(.20u,0.20u);
       rung  := (-.20u,0.10u)--(.20u,0.10u);

       picture symbol,test_symbol;
       symbol:= image (
                  thdraw sideL;
                  thdraw sideR;
                  thdraw rung;
                  );

       forever:
             t := arctime cas of P;
             thdraw symbol shifted (point t of P) withcolor black;
             cas := cas + mojkrok;
             exitif cas > dlzka + (mojkrok / 3); % for rounding errors
       endfor;

     enddef;