protected function expandConversionMapping(array $mapping)
    {
        $ret = [];
        $matches = [];
        foreach ($mapping as $m) {
            if (preg_match('/^(.*)\s*=>\s*(.*)/', $m, $matches)) {
                $ret[trim($matches[1])] = trim($matches[2]);
            }
        }
        return $ret;
    }