public function get(string $primary_key, $sub_key)
    {
        if (!$this->enabled) {
            return; // Not possible.
            //
        } elseif (!($key = $this->key($primary_key, $sub_key))) {
            return; // Fail; e.g., race condition.
        }
        $value = $this->Pool->get($key); // Possibly `false`.
        // See: <http://php.net/manual/en/memcached.get.php#92275>

        if ($this->Pool->getResultCode() === \Memcached::RES_SUCCESS) {
            return $value; // Return the value.
        }
    }