optional arguments in math. how to?

[Amir Seginer]

Hello,

I've just been trying use the command \xrightarrow with it optional 
argument, e.g.

\[ leftside \xright[down]{up} rightside \]

Is it possible to do this (and in general use optional arguments in 
math/display mode)? I've tried writing it as regular text (without the 
\[ and \]) and turning it to a math/display mode. I've also tried to 
insert it manually in the .lyx file, but to no avail.

I did manage to use it in an ERT box, but I would rather not write the 
whole equation that way.

If it is indeed imposible, might it be better if there was an option 
within math mode to enter pure latex, without Lyx "helping" in the 
middle. Something like the ERT box in the regular text. I think  (if I 
remember correctly) that it was possible in earlier version 1.1.6 (or so).

Thanks for any help,

Amir.

P.S.

I'm using Lyx 1.2.1 (Xforms)