This method compiles an
XPathExpression which can then be used for (repeated) evaluations.
xpathExpr = document.createExpression(xpathText, namespaceURLMapper);
xpathText(the XPath expression to be compiled)
namespaceURLMapper(maps a namespace prefix to a namespace URL (or null if none needed))
Prior to Firefox 3, you could call this method on documents other than the one you planned to run the XPath against. Under Firefox 3, you must call it on the same document.