runMPgraphicstrue produces undefined figure

[Sanjoy Mahajan <sanjoy@mrao.cam.ac.uk>]

I'm playing with simple files in order to learn metafun.  The following
small example produces an undefined figure.  It illustrates either a
context problem or, more likely, a confusion on my part.

================= cut here ===========================================
\runMPgraphicstrue
\starttext

\startreusableMPgraphic{a}
fill fullcircle scaled 2cm;
\stopreusableMPgraphic

\placefigure[here,none]{}{\reuseMPgraphic{a}}
\input tufte
\stoptext
================= cut here ===========================================

The figure is a square saying 'undefined'.  Taking away the
\runMPgraphicstrue line produces the expected filled circle.

I use a web2c distribution (linux) with

  context version 2006.08.08 21:51,
  TeXExec | version 6.2.0 - 1997-2006 - PRAGMA ADE/POD

It passes the write18 test given in the metafun-s.pdf manual (p.214 of
the PDF file).

-Sanjoy

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