arabluatex: Underlining an Arabic text

Question

How to fix the following underlined Arabic text?

\documentclass{article}
\usepackage[tuenc]{fontspec}

\newfontfamily\arabicfont{Amiri}[Script=Arabic, RawFeature={+anum}]
\usepackage{arabluatex}

\begin{document}

Without underlining:

\begin{txarab}
    كتاب
\end{txarab}

With underlining

\begin{txarab}
\underline{كتاب}
\end{txarab}

\end{document}

Answer 1

\underline knows nothing about arabic text. Use:

\begin{txarab}
\underline{\begin{txarab}كتاب\end{txarab}}
\end{txarab}

or define your own \arabuline which does the \begin .. \end automatically

Answer 2

There might be different solutions, but I find the following one working using ulem package:

\documentclass{article}
\usepackage[tuenc]{fontspec}

\newfontfamily\arabicfont{Amiri}[Script=Arabic, RawFeature={+anum}]
\usepackage{arabluatex,ulem}

\begin{document}

Without underlining:

\begin{txarab}
    كتاب
\end{txarab}

With underlining

\begin{txarab}
\uline{كتاب}
\end{txarab}

\end{document}