There is something in the processing of mathml I do not understand.
I keep the xml-node in a
  \startxmlsetups my:setup
  \def\mynode{#1}
  \stopxmlsetups

Doing \xmlflush{\mynode} does typeset the contents of mynode, but not when mynode contains a mathml tree! Then the error is:

Undefined control sequence ...
\MMLhack ->\let \MMLpar
                        \par \let \par \relax \everyvbox {\let \par \MMLpar }
\12>:mml:math ...}} {\automathematics }}{\MMLhack
                                                  \xmlflush {#1}}\endgroup

Tested it with a simple:
  <math xmlns='http://www.w3c.org/mathml' version='2.0'>
  <apply><root/> <ci> x </ci> </apply>
  </math>

The strange thing is, when putting the mathlml tree insize <text> mathml-content </text> where:
  \startxmlsetups xam:text
  \xmltrim{#1}\par
  \stopxmlsetups
then typesetting goes without an error!

Can someone explain this behaviour?

Hans van der Meer