docs: def update(self, *args, **kwargs):
          '''
          Updates the set to include all arguments passed in. If the keyword
          argument preprocess is passed, then each element is preprocessed
          before being added.
          '''
          preprocess = kwargs.get('preprocess')
          for s in args:
              for e in s:
                  self._dict_set(preprocess(e) if preprocess else e, True)