private function _scan(&$fromDir, &$toDir, &$files, &$dirs)
    {
        /* mappers should be generic, so we get the mappers here and
        pass them on to builMap. This method is not redundan like it seems */
        $mapper = $this->getMapper();

        $this->buildMap($fromDir, $toDir, $files, $mapper, $this->fileCopyMap);

        if ($this->includeEmpty) {
            $this->buildMap($fromDir, $toDir, $dirs, $mapper, $this->dirCopyMap);
        }
    }