[XeTeX] Problem with Polyglossia's \aemph command

[Gareth Hughes]

On the attached minimal example I get the following error when I use the
Polyglossia command \aemph to place an overline over text.

> ! Undefined control sequence.
> \aemph #1->$\overline {\hboxR 
>                               {#1}}$
> l.6 ܗܢܐ ܛܣܛ: \aemph{ܫܠܡܐ}
>                                    
> The control sequence at the end of the top line
> of your error message was never \def'ed. If you have
> misspelled it (e.g., `\hobx'), type `I' and the correct
> spelling (e.g., `I\hbox'). Otherwise just continue,
> and I'll forget about whatever was undefined.
> 
> Missing character: There is no ܫ in font cmmi10!
> Missing character: There is no ܠ in font cmmi10!
> Missing character: There is no ܡ in font cmmi10!
> Missing character: There is no ܐ in font cmmi10!

Why can't I get this work?

Gareth.
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