code: public String getXPath(NamespaceContext context, String xml, String xPathExpr) {
          return (String) evaluateXpath(context, xml, xPathExpr, null);
      }
docs: Evaluates xPathExpr against xml, returning single match.
  @param xml xml document to apply XPath to.
  @param xPathExpr XPath expression to evaluate.
  @return result of evaluation, null if xml is null.