[XXE] XPath subset for parent in elementTemplate?

[Bill Fenner]

Does the XPath subset for parent in elementTemplate permit selecting
by attribute?  In the xml2rfc format, <t> has an optional "hangText"
attribute if the parent <list> has style="hanging"; I tried

  <elementTemplate name="hanging" selectable="override" xmlns=""
   parent="li...@style='hanging']">
  <t hangText="???"> </t></elementTemplate> 

but xxe gave the "missing or invalid parent" error on startup; I see
in the configure documentation that parent supports "XPath (subset)"
but it's not clear what the subset might be.

Thanks,
  Bill