[MiKTeX] When did btex..etex break in MetaPost?

[Geoff R. <mi...@ge...>]

Hi all,

Just recently I've noticed that commands such as
	label.rt(btex $y=x^2$ etex, (20, 20))

are failing with this enigmatic description:
--------
 >> E:\School\Maths\TeacherTraining\integration2.mp
 >> E:\School\Maths\TeacherTraining\integration2.mpx
! Unable to make mpx file.
l.62     label.rt(btex
                        $y=x^2$ etex, (maxx*dx,fn(maxx)*dy));
The two files given above are one of your source files
and an auxiliary file I need to read to find out what your
btex..etex blocks mean. If you don't know why I had trouble,
try running it manually through MPtoTeX, TeX, and DVItoMP
--------
I'm pretty handy with MetaPost and really want it work again - anybody got 
any ideas?

Thanks