protected function findImport($url) {
        $urls = array();

        // for "normal" scss imports (ignore vanilla css and external requests)
        if (!preg_match('/\.css|^http:\/\/$/', $url)) {
            // try both normal and the _partial filename
            $urls = array($url, preg_replace('/[^\/]+$/', '_\0', $url));
        }

        foreach ($this->importPaths as $dir) {
            if (is_string($dir)) {
                // check urls for normal import paths
                foreach ($urls as $full) {
                    $full = $dir .
                        (!empty($dir) && substr($dir, -1) != '/' ? '/' : '') .
                        $full;

                    if ($this->fileExists($file = $full.'.scss') ||
                        $this->fileExists($file = $full))
                    {
                        return $file;
                    }
                }
            } else {
                // check custom callback for import path
                $file = call_user_func($dir,$url,$this);
                if ($file !== null) {
                    return $file;
                }
            }
        }

        return null;
    }