code: def assert_no_union(self, collection1, collection2,
                          failure_message='Expected no overlap between collections: "{}" and "{}"'):
          
          assertion = lambda: len(set(collection1).intersection(set(collection2))) == 0
          failure_message = unicode(failure_message).format(collection1, collection2)
          self.webdriver_assert(assertion, failure_message)
docs: Asserts that the union of two sets is empty (collections are unique)