Update the set, adding elements from all *iterables*.

def update(self, *iterables):
        """Update the set, adding elements from all *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