"""Internal mutation token. Only pixel_cursor.* should import this module."""
from __future__ import annotations


class _Token:
    __slots__ = ()

    def __repr__(self) -> str:
        return "<pixel_cursor mutation token>"


_MUTATION_TOKEN = _Token()


def _verify(token: object) -> None:
    if token is not _MUTATION_TOKEN:
        raise PermissionError(
            "Artifact construction requires the pixel_cursor mutation token. "
            "All Artifact creation and mutation must go through a PixelCursor "
            "(see pixel_cursor.open_cursor and cursor.write_* methods)."
        )