code: def update(self, *iterables):
          
          _set = self._set
          values = set(chain(*iterables))
          if (4 * len(values)) > len(_set):
              _list = self._list
              _set.update(values)
              _list.clear()
              _list.update(_set)
          else:
              _add = self.add
              for value in values:
                  _add(value)
          return self
docs: Update the set, adding elements from all *iterables*.