texexec and TEX

[Michal Kvasnicka <quasar@econ.muni.cz>]

Good evening.

Is there some restriction to Metapost macros when compiled by texexec 
--mptex? I'd like to compile a file like this:

% test
input TEX;

beginfig(1)
  z1=(0,1cm);
  z2=(3cm,1cm);

  dotlabel.top(btex $z_1$ etex,z1);
  dotlabel.top(btex $z_2$ etex,z2);
  draw TEX("$\underbrace{\hbox to 
"&decimal(xpart(z2)-xpart(z1))&"pt{\hss}}$") shifted z1;
endfig;

end.

But I've got a following error:
This is MetaPost, Version 0.641 (Web2C 7.3.1)
(TEXuse.mp (/usr/share/texmf/metapost/base/TEX.mp) (mptextmp.mp
 >> mptextmp.mp
 >> mptextmp.mpx
! Unable to make mpx file.
l.1 btex
         $\underbrace{\hbox to 85.03935pt{\hss}}$ etex
Transcript written on TEXuse.log.
 error in metapost run : TEXuse.mp:1

What's wrong? How can I improve it?

Many thanks for any hint.
M.K.

P.S. I use TeXExec 3.3 with Metapost Version 0.641 (Web2C 7.3.1) under 
Linux (teTeX).