[metapost] verbatimtex not used from an .mp include file?

[Sanjoy Mahajan]

Do commands defined in verbatimtex...etex  have to be defined
in the file that uses them?  Here's an example that puzzles me:

  $ cat blah.mp
  input t2
  beginfig(1)
  label.bot(btex \x etex, origin);
  endfig;
  end
  $ cat t2.mp
  verbatimtex
  \def\x{abc}
  etex

So blah.mp should know about \x, but running metapost produces this:

  $ mpost blah
  This is MetaPost, Version 0.901 (Web2C 7.5.3)
  (blah.mp (t2.mp)/usr/bin/makempx: Command failed: tex --parse-first-line mpxerr.tex; see mpxerr.log

  >> blah.mp
  >> blah.mpx
  ! Unable to make mpx file.
  l.7 label.bot(btex
		     \x etex, origin);
  Transcript written on blah.log.

The relevant piece from mpxerr.log is:

  (./mpx18577.tex
  ! Undefined control sequence.
  <argument> \x 

  \mpxshipout ...\shipout \hbox {\setbox 0=\hbox {#1
						    }\dimen 0=\ht 0 \advance \...
  l.9 \x}


-Sanjoy

`A society of sheep must in time beget a government of wolves.'
   - Bertrand de Jouvenal