code: def add_css_class(css_classes, css_class, prepend=False):
      
      classes_list = split_css_classes(css_classes)
      classes_to_add = [c for c in split_css_classes(css_class) if c not in classes_list]
      if prepend:
          classes_list = classes_to_add + classes_list
      else:
          classes_list += classes_to_add
      return " ".join(classes_list)
docs: Add a CSS class to a string of CSS classes