private function parseApiVersion ($responseHeaders)
    {
        // A header that's unique to the legacy API
        if (array_key_exists('X-SODA2-Legacy-Types', $responseHeaders) && $responseHeaders['X-SODA2-Legacy-Types'])
        {
            return 1;
        }

        // A header that's unique to the new API
        if (array_key_exists('X-SODA2-Truth-Last-Modified', $responseHeaders))
        {
            if (empty($this->metadata))
            {
                $this->getMetadata();
            }

            if ($this->metadata['newBackend'])
            {
                return 2.1;
            }

            return 2;
        }

        return 0;
    }