protected function cacheUnset(string $primary_key, $sub_key)
    {
        if ($primary_key === 'x___keys' || $primary_key === 'x___refs') {
            throw $this->c::issue('Attempting to unset a reserved primary key.');
        }
        $sub_key                                 = (string) $sub_key;
        $this->x___cache[$primary_key][$sub_key] = null;
        unset($this->x___cache[$primary_key][$sub_key]);
    }