def __init__(self, bases, pv=None, *, force=False):
        self.bases = list(bases)
        if self.size > self._size_max and not force:
            raise ValueError(
                'Density matrix of the system is going to have {} items. It '
                'is probably too much. If you know what you are doing, '
                'pass `force=True` argument to the constructor.')