def do_xref(self, node):
id = node.attributes["<STR_LIT:id>"].value<EOL>self.parse(self.randomChildElement(self.refs[id]))<EOL>
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'>.