docs: def count_elements_exactly_by_selector(self, number, selector):
      """
      Assert n elements exist matching the given selector.
      """
      elems = find_elements_by_jquery(world.browser, selector)
      number = int(number)
      if len(elems) != number:
          raise AssertionError("Expected {} elements, found {}".format(
              number, len(elems)))