protected function key(string $primary_key, $sub_key): string
    {
        if (!$this->enabled) {
            return ''; // Not possible.
            //
        } elseif (!($namespaced_primary_key = $this->nspKey($primary_key))) {
            return ''; // Not possible; e.g., empty key.
        }
        $namespaced_primary_key_uuid = ''; // Initialize.

        do { // Avoid race issues.
            $attempts = $attempts ?? 0;
            ++$attempts; // Counter.

            if (($existing_namespaced_primary_key_uuid = (string) $this->Pool->get($namespaced_primary_key))) {
                $namespaced_primary_key_uuid = $existing_namespaced_primary_key_uuid;
                break; // All good; stop here.
            }
            if (!isset($new_namespaced_primary_key_uuid)) {
                $new_namespaced_primary_key_uuid = $this->c::uuidV4();
            }
            if ($this->Pool->add($namespaced_primary_key, $new_namespaced_primary_key_uuid)) {
                $namespaced_primary_key_uuid = $new_namespaced_primary_key_uuid;
                break; // All good; stop here.
            }
            $result_code = $this->Pool->getResultCode();
            //
        } while ($attempts < $this::MAX_WRITE_ATTEMPTS && $result_code === \Memcached::RES_NOTSTORED);

        if (!$namespaced_primary_key_uuid) {
            return ''; // Failure; e.g., race condition.
        }
        $sub_key                            = (string) $sub_key;
        $namespaced_primary_key_uuid_prefix = $namespaced_primary_key_uuid.'\\';
        $namespaced_key                     = $namespaced_primary_key_uuid_prefix.$sub_key;

        if (isset($namespaced_key[251])) {
            throw $this->c::issue(sprintf('Sub key too long; %1$s bytes max.', 250 - strlen($namespaced_primary_key_uuid_prefix)));
        }
        return $namespaced_key;
    }