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.