[XeTeX] underline problem in arabxetex

[Mehdi Omidali]

There is a problem with underline in math mode with arabxetex. I comiled the
following lines

     \documentclass{article}
     \usepackage{arabxetex}
     \begin{documnet}
     \begin{farsi}
     $$\underline{I}$$
     \end{farsi}
     \end{document}
and got the following error:
! Missing $ inserted.
<inserted text> 
                $
l.5 $$\underline{I}
                   $$
I've inserted a begin-math/end-math symbol since I think
you left one out. Proceed, with fingers crossed.

! Missing } inserted.
<inserted text> 
                }