[prodigal_erik]

XPath is far too concise. It needs an XML encoding. Something like PMML, where math looks like

  <DefineFunction name="frob" optype="continuous"> 
    <ParameterField name="x" optype="continuous"/> 
    <Apply function="+">
      <FieldRef field="x"/>
      <Constant>42</Constant>
    </Apply>
  </DefineFunction>

I wish I were joking.

[ewiethoff]

Content MathML to the rescue: http://en.wikipedia.org/wiki/MathML#Content_MathML