Map<String, String> ns = Collections.singletonMap("x", "http://example.org");
XPathExpectationsHelper helper = new XPathExpectationsHelper("//b", ns);
WebServiceMessageMatcher matcher = helper.exists();
assertNotNull(matcher);
WebServiceMessage message = createMock(WebServiceMessage.class);
expect(message.getPayloadSource())
.andReturn(new StringSource("<a:a xmlns:a=\"http://example.org\"><a:b/></a:a>")).times(2);
replay(message);
matcher.match(message);