code: public static void assertXpathValuesNotEqual(String controlXpath,
                                                   Document controlDocument,
                                                   String testXpath,
                                                   Document testDocument)
          throws XpathException {
          XpathEngine xpath = XMLUnit.newXpathEngine();
          String control = xpath.evaluate(controlXpath, controlDocument);
          String test = xpath.evaluate(testXpath, testDocument);
          if (control!=null) {
              if (control.equals(test)) {
                  fail("Expected test value NOT to be equal to control but both were "
                       + test);
              }
          } else if (test == null) {
              fail("Expected test value NOT to be equal to control but both were empty");
          }
      }
docs: Assert that the evaluation of two Xpaths in two documents are
  NOT equal
  @param controlXpath
  @param testXpath
  @param controlDocument
  @param testDocument