Truclient Ajax FF - document.evaluate does not work; complains that XPathResult is not defined

I am trying to use document.evaluate to obtain a reference to a set of elements via use of an XPath expression.

Whenever TruClient runs the step, the script stops with the same error:
 ReferenceError: XPathResult is not defined

I have tried several code permutations but always get the same result.  I have asked HP Support to confirm that document.evaluate and XPathResult are both supported.

In the meantime, can anyone say why this will not work?

//----------------------------------------------------------------------
//Permutation 1
 var professions =
 document.evaluate(
 "//a[@target='_self']", document, null, XPathResult.ANY_TYPE, null
 );

 //Permutation 2
 var xpr;
 document.evaluate(
 "//a[@target='_self']", document, null, XPathResult.ANY_TYPE, xpr
 );

 //Permutation 3
 var xpr = new XPathResult();
 document.evaluate(
 "//a[@target='_self']", document, null, XPathResult.ANY_TYPE, xpr
 );

 //Permutation 4
 var xpr = XPathResult.create();
 document.evaluate(
 "//a[@target='_self']", document, null, XPathResult.ANY_TYPE, xpr
 );
//----------------------------------------------------------------------