protected function &cacheGet(string $primary_key, $sub_key)
    {
        if ($primary_key === 'x___keys' || $primary_key === 'x___refs') {
            throw $this->c::issue('Attempting to get a reserved primary key.');
        }
        $sub_key = (string) $sub_key; // Force string.

        if (isset($this->x___cache[$primary_key][$sub_key])) {
            return $this->x___cache[$primary_key][$sub_key];
        } else {
            $this->x___cache[$primary_key][$sub_key] = null;
            return $this->x___cache[$primary_key][$sub_key];
        }
    }