<div dir="ltr">$\underline{X}$ gives error because the definition of \underline in bidi.sty is buggy.<br><br>The definition should be:<br><br>%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%<br>\let\@@@underline=\underline<br>
\def\underline#1{%<br>&nbsp; \relax<br>&nbsp; \ifmmode\@@@underline{#1}%<br>&nbsp; \else<br>\if@rl $\@@@underline{\hbox{\RL{#1}}}\m@th$\relax<br>\else<br>$\@@@underline{\hbox{#1}}\m@th$\relax\fi\fi}<br><br><br><br><br>%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%<br>
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