public function addPrefix($prefix, $paths)
    {
        if (!$prefix) {
            foreach ((array) $paths as $path) {
                $this->fallbackDirs[] = $path;
            }

            return;
        }
        if (isset($this->prefixes[$prefix])) {
            $this->prefixes[$prefix] = array_merge(
                $this->prefixes[$prefix],
                (array) $paths
            );
        } else {
            $this->prefixes[$prefix] = (array) $paths;
        }
    }