question about MPpositiongraphic

["Thomas A. Schmitz" <thomas.schmitz@uni-bonn.de>]

Hi all,

first, best wishes for the New Year to all of you - may 2009 bring  
peace and fortune and world domination for ConTeXt!

And my little question - trivial for the resident experts, I guess,  
but I can't find a solution: I want to write a little macro that will  
allow me to place a thin curved line under letters (metrical symbol  
for synizesis). Now that metafun is so fast in mkiv, I thought I'd use  
a MPpositiongraphic for that purpose, but my problem is that I can't  
make the graphic really local - when it is used more than once in a  
document, Metafin uses the last values for all occurrences. I tried  
grouping and the "save" command, but to no avail. Here's an example  
that shows the problem:

\startMPpositiongraphic{placesynizesis}
      StartPage ;
      begingroup ; save a ;
      initialize_box(\MPpos{\MPvar{self}}) ;
      numeric a ; a = ExHeight/6 ;
      z1 = llxy ;
      z2 = lrxy ;
      z3 = 1/2[z1,z2] ;
      z4 = ((x1 + 2a),(y1 - 2a)) ;
      z5 = (x3, (y3 - 4a)) ;
      z6 = ((x2 - 2a),(y2 - 2a)) ;
      path syn ; syn := z4 .. z5 .. z6 ;
      pickup pencircle scaled a ;
      draw syn ;
      anchor_box(\MPanchor{\MPvar{self}}) ;
      endgroup ;
      StopPage ;
\stopMPpositiongraphic%
\setMPpositiongraphic{POS}{placesynizesis}

\define[1]\synizesis
   {\hpos{POS}{#1}}

\starttext

\switchtobodyfont[25pt]

test: \synizesis{au} and \synizesis{aeiou}

\stoptext

Any solution to this conundrum?

All best

Thomas
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