code: def join_css_class(css_class, *additional_css_classes):
      
      css_set = set(chain.from_iterable(
          c.split(' ') for c in [css_class, *additional_css_classes] if c))
      return ' '.join(css_set)
docs: Returns the union of one or more CSS classes as a space-separated string.
      Note that the order will not be preserved.