table and MPpositiongraphic

[Patrick Gundlach <pg@levana.de>]

Hello, 

while browsing thru the source and trying some examples, I can't
sensible output of the following code. (No MP stuff is visible)

%output=pdf

\setupcolors[state=start]

\def\cw#1{\color[green]{#1}}

\def\TableShade#1#2%
  {\startMPpositiongraphic
     initialize_area(\MPpos{b:#1},\MPpos{e:#1}) ;
     show pxy;
     linear_shade(pxy,0,.4#2,.9#2) ;
     anchor_area(\MPanchor{b:#1}) ;
   \stopMPpositiongraphic}

\dosetpositionaction{b:x}{\TableShade{x}{red}}
\dosetpositionaction{b:y}{\TableShade{y}{green}}
\dosetpositionaction{b:z}{\TableShade{z}{blue}}

\starttext
\starttable[|c|c|c|]
\GFC[b:z] \cw{this is a small}  \NC       \cw{table}            \NC     in which we      \NC \FR
\NC       \cw{will demonstrate} \GFC[c:y] \cw{that this}        \GTC[c] \cw{positioning} \NC \MR
\GSC[a:x] \cw{mechanism also}   \GTC[b]   \cw{works quite well} \NC     in tables        \NC \LR
\stoptable

\stoptext

Any ideas how to get the MPpositiongraphic working?

Patrick