code: def do_xref(self, node):
          
          id = node.attributes["id"].value
          self.parse(self.randomChildElement(self.refs[id]))
docs: handle <xref id='...'> tag
          
          An <xref id='...'> tag is a cross-reference to a <ref id='...'>
          tag.  <xref id='sentence'/> evaluates to a randomly chosen child of
          <ref id='sentence'>.