does metafun require texexec --final?

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

I was trying experiments with the file below, changing r from 1cm to
10cm or vice versa, then rerunning texexec and looking at the pdf.  But
the pdf changes only the second time that I run texexec (and each time,
texexec runs metafun for the embedded graphic).  However, the pdf
changes right away if I give --final to texexec.  

So, is --final a requirement when using metafun, or am I hacking around
a problem I've caused by leaving something out?

Oh, having heeded Aditya's earlier advice on the list: \write18 is
enabled.

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

\startreusableMPgraphic{a}
r := 1cm;
fill fullcircle scaled r;
\stopreusableMPgraphic

\placefigure[force,none]{}{\reuseMPgraphic{a}}

the figure above should be a small circle

\stoptext
=============== cut here ==========

-Sanjoy