public function addPart($value, $isEncoded, $index)
    {
        if (empty($index)) {
            $index = 0;
        }
        if ($isEncoded) {
            $this->extractMetaInformationAndValue($value, $index);
        } else {
            $this->literalParts[$index] = $this->convertEncoding($value);
        }
    }