Trouble making basic MetaPost graphics using ConTeXt Minimals

["Kevin D. Robbins" <krobbins@alumni.princeton.edu>]

Using a fresh install of ConTeXt Minimals today (ConTeXt version
2011.07.14 16:09, luatex version beta-0.70.1-2011051918) I am trying
to build the following MetaPost graphics:

---8<--- file: test.mp
fontmapfile "=lm-math.map" ;

beginfig(1)
  draw fullcircle scaled 3cm ;
endfig ;

beginfig(2) ;
  fill fullcircle scaled 5cm withcolor red ;
endfig ;

beginfig(3);
  numeric u;
  u = 1cm;
  draw (0,2u)--(0,0)--(4u,0);
  pickup pencircle scaled 1pt;
  draw (0,0){up}
    for i=1 upto 8: ..(i/2,sqrt(i/2))*u endfor;
  label.lrt(btex $\sqrt x$ etex, (3,sqrt 3)*u);
  label.bot(btex $x$ etex, (2u,0));
  label.lft(btex $y$ etex, (0,u));
endfig;

end .
---8<---

Building the file using "mpost test.mp" I get the following:

This is MetaPost, version 1.504 (kpathsea version 6.0.0)
(mpost.mp (/opt/tools/ConTeXt/beta/tex/texmf/metapost/base/plain.mp
Preloading the plain mem file, version 1.004))
(./test.mp{pdftex.map}{lm-math.map} [1]
[2]/opt/tools/ConTeXt/beta/tex/texmf-context/scripts/context/ruby/base/switch.rb:570:in
`locateseries': undefined method `each' for "tex":String
(NoMethodError)
	from /opt/tools/ConTeXt/beta/tex/texmf-context/scripts/context/ruby/base/switch.rb:397:in
`block in expand'
	from /opt/tools/ConTeXt/beta/tex/texmf-context/scripts/context/ruby/base/switch.rb:392:in
`each'
	from /opt/tools/ConTeXt/beta/tex/texmf-context/scripts/context/ruby/base/switch.rb:392:in
`expand'
	from /opt/tools/ConTeXt/beta/tex/texmf-context/scripts/context/ruby/texexec.rb:789:in
`<main>'


>> test.mp
>> test.mpx
! Unable to make mpx file.
l.18   label.lrt(btex
                      $\sqrt x$ etex, (3,sqrt 3)*u);
2 output files written: test.1 .. test.2

The first two graphics are produced correctly, but there is no output
for the third graphic with the labels.

Building the file using "texexec --mptex test.mp" all three graphics
are produced, but the label $\sqrt x$ does not produce the square root
symbol.

Am I doing something obviously wrong? Can anyone reproduce this?

Thanks,

Kevin
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