def get_unused_options(cfg_dict, options):
    """
    Returns the unused keys in a configuration dictionary. An unused key
    is a key that is not seen in *options*.
    
    Parameters
    ----------
    cfg_dict : dict
    options : :py:class:`~enrich2.plugins.options.Options`

    Returns
    -------
    `set`
        Set of unused keys.
    """
    unused = set()
    option_keys = set(options.keys())
    for key in cfg_dict.keys():
        if key not in option_keys:
            unused.add(key)
    return unused