Make an option unmutable.

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

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

def lock_option(self, key, subkey):
        """Make an option unmutable.

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

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

        self.gc.loc[
            (self.gc["k1"] == key) &
            (self.gc["k2"] == subkey), "locked"] = True