[NTG-context] Scope of BodyFontSize etc.

[Oliver Buerschaper]

Dear wizards of ConTeXt,

I'm wondering why ConTeXt doesn't seem to know about "BodyFontSize" in  
the "MPinclusions" block … try:

---
\startMPinclusions
     DefaultRuleThickness := 0.47998pt*BodyFontSize/12pt;

     pickup pencircle scaled DefaultRuleThickness;
     defaultpen := savepen;
\stopMPinclusions

\startuseMPgraphic{localRule1a}
     draw fullsquare scaled BodyFontSize;
\stopuseMPgraphic


\starttext

\startformula
     \Phi(\useMPgraphic{localRule1a})
\stopformula

\stoptext
---

As soon as I move the content of "MPinclusions" over to the beginning  
of my "useMPgraphic" code everything works as expected …

Where's the best location to put this sort of definitions then? Is it  
the "MPenvironment" block?

Oliver