import typing


def thumbnail_url(row: typing.Mapping[str, str]) -> typing.Optional[str]:
    """A thumbnail URL.

    Args:
        row: An input CSV record.

    Returns:
        The thumbnail URL.
    """
    if row.get("Thumbnail URL"):
        return row["Thumbnail URL"]

    if row.get("IIIF Access URL"):
        return row["IIIF Access URL"] + "/full/!200,200/0/default.jpg"

    return None