How to use the parsel.lxmldocument.LxmlDocument function in parsel

def test_caching(self):
        r1 = HtmlResponse('http://www.example.com', body='')
        r2 = r1.copy()

        doc1 = LxmlDocument(r1)
        doc2 = LxmlDocument(r1)
        doc3 = LxmlDocument(r2)

        # make sure it's cached
        assert doc1 is doc2
        assert doc1 is not doc3

def test_null_char(self):
        # make sure bodies with null char ('\x00') don't raise a TypeError exception
        body = 'test problematic \x00 body'
        response = TextResponse('http://example.com/catalog/product/blabla-123',
                                headers={'Content-Type': 'text/plain; charset=utf-8'},
                                body=body)
        LxmlDocument(response)

def test_caching(self):
        r1 = HtmlResponse('http://www.example.com', body='')
        r2 = r1.copy()

        doc1 = LxmlDocument(r1)
        doc2 = LxmlDocument(r1)
        doc3 = LxmlDocument(r2)

        # make sure it's cached
        assert doc1 is doc2
        assert doc1 is not doc3

def test_caching(self):
        r1 = HtmlResponse('http://www.example.com', body='')
        r2 = r1.copy()

        doc1 = LxmlDocument(r1)
        doc2 = LxmlDocument(r1)
        doc3 = LxmlDocument(r2)

        # make sure it's cached
        assert doc1 is doc2
        assert doc1 is not doc3