[XeTeX] translating CSs into Persian (New XeTeX primitive required)

[Joseph Wright]

وفا خلیقی wrote:
> Hi Jonathan
> 
> Today, I was playing with control sequences in Plain TeX and LaTeX and was
> translating them into Persian using \let and \def control sequences. But
> when it comes to \let and \def themselves, this approach does not make any
> sense and in fact has side effects.

Just wondering in what way this fails?  Can we see an example?
-- 
Joseph Wright