public function &checkProcess(string $url, string $sha1)
    {
        if (is_string($sri = &$this->cacheGet('sris', $sha1))) {
            return $sri; // Cached this already.
        } else { // Set in-process cache to `null`.
            $sri = null; // Not in the cache.
            return $sri; // By reference.
        }
    }