public function checkMemcache(string $url, string $sha1)
    {
        if (!$this->memcache_enabled) {
            return $sri = null; // Not possible.
        } elseif (is_string($sri = $this->c::memcacheGet('sris', $sha1))) {
            return $sri; // Ideal; this avoids disk IO.
        }
        return $sri = null; // Not in the cache.
    }