def _coalesce_version_id(*args):
    """Helper to coalesce a list of version_ids down to one"""
    version_ids = set(args)
    if None in version_ids:
        version_ids.remove(None)
    if len(version_ids) > 1:
        raise ValueError(
            "Cannot coalesce version_ids where more than one are defined,"
            " {}".format(version_ids)
        )
    elif len(version_ids) == 0:
        return None
    else:
        return version_ids.pop()
Helper to coalesce a list of version_ids down to one
s3fs/core.py