def METHOD_NAME(self, *args):
    """Unions the equivalence classes containing the elements in `*args`."""
    if self._readonly:
        raise AttributeError
    if len(args) == 0:
        return None
    if len(args) == 1:
        return self[args[0]]
    for a, b in zip(args[:-1], args[1:]):
        result = self._union_pair(a, b)
    return result