[metapost] Limits for TEX.mp?

[Brian Kell]

I am working on a couple of MetaPost drawings that have a lot of nearly 
identical labels, differing only in subscripts, so I'm generating them 
with the TEX() macro from TEX.mp inside a pair of nested loops. These 
are all very simple labels---they're all of the following form:
     TEX("$c_{" & decimal(i) & decimal(j) & "}$")

Partway through my second drawing, I get the error "Unable to make mpx 
file" from MetaPost, on a loop iteration that is just like all the other 
iterations. The full error is shown below.

fatal: Command failed: etex --parse-first-line --interaction=nonstopmode 
mp632926.tex; see mpxerr.log
 >> mptextmp.mp
 >> mptextmp.mpx
! Unable to make mpx file.
l.1 btex
          $c_{31}$ etex
1 output file written: final-3a.mps
Transcript written on final.log.

It seems that MetaPost is running into a limit on the number of times 
the TEX() macro can be called. For this particular file, the limit 
appears to be 122 times (the error occurs on the 123rd call).

I was able to isolate the problem to a simpler input file:

input TEX;
beginfig(1);
for i=1 upto 300:  label(TEX(decimal(i)), (0,0));  endfor
endfig;

When I feed this to MetaPost, everything goes smoothly for the first 243 
iterations, but I get the "Unable to make mpx file" error on the 244th 
iteration. I can make the label text a little more complex to more 
closely mirror my original file:

input TEX;
beginfig(1);
for i=1 upto 300:
     label(TEX("$x_{" & decimal(i) & "}$"), (0,0));
endfor
endfig;

In this case I get 122 successful iterations, but the "Unable to make 
mpx file" error occurs on the 123rd iteration, which is exactly where I 
see the problem in my original file.

I can work around this issue for the moment by separating my two 
drawings into two different MetaPost files, because individually the 
drawings use fewer than 123 TEX() calls each. However, I can foresee 
that someday I might want to make a drawing with more labels than that; 
is there something I can do to fix this problem, or at least raise the 
limit?

I'm using MetaPost version 1.208 (kpathsea version 5.0.0).

Brian Kell