According to the DFDL spec, fn:exactly-one returns "True if the provided sequence contains exactly one node/value," which suggests that the function would return false if the sequence contains > or < 1 node/value. The XPath description states that fn:exactly-one "returns $arg if it contains exactly one item. Otherwise, raises an error." It seems of value to have a true/false result, because something that raises an error like this can only be used as an assertion effectively. On the other hand, that's not what XPath does. Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com Please note: Contributions to the DFDL Workgroup's email discussions are subject to the OGF Intellectual Property Policy <http://www.ogf.org/About/abt_policies.php>