def _rs1024_verify_checksum(data: Indices, extendable_backup_flag: bool) -> bool:
    """
    Verifies a checksum of the given mnemonic, which was already parsed into Indices.
    """
    return (
        _rs1024_polymod(tuple(_get_customization_string(extendable_backup_flag)) + data)
        == 1
    )

    Verifies a checksum of the given mnemonic, which was already parsed into Indices.
    
electrum/slip39.py