Get list of available values for an option.

        :param str key: First identifier of the option.
        :param str subkey: Second identifier of the option.

        :return: Union[:func:`list`, :data:`None`] - alternative values
            for the option, if any specified (otherwise, is open).

        :raise:
            :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                option.

def get_option_alternatives(self, key, subkey):
        """Get list of available values for an option.

        :param str key: First identifier of the option.
        :param str subkey: Second identifier of the option.

        :return: Union[:func:`list`, :data:`None`] - alternative values
            for the option, if any specified (otherwise, is open).

        :raise:
            :NotRegisteredError: If ``key`` or ``subkey`` do not define any
                option.
        """
        key, subkey = _lower_keys(key, subkey)
        _entry_must_exist(self.gc, key, subkey)

        return self.gc[(self.gc["k1"] == key) &
                       (self.gc["k2"] == subkey)]["values"].values[0]