import schema namespace ipo="http://www.example.com/IPO";

declare function local:comment-text($c as element(ipo:comment))
  as xs:string
{
    xs:string( $c )
};

for $p in doc("ipo.xml")//element(ipo:purchaseOrder),
    $t in local:comment-text( $p//ipo:shipComment )
where $p/shipTo/name="Helen Zoe"
    and $p/orderDate = xs:date("1999-12-01")
return $t