[XeTeX] Can not Set Italic maths characters in plain XeTeX

[VAFA KHALIGHI]

I studied Mr.Kew's article at EuroBachoTeX 2007, where as a part of article
he explained how to set Camria Math as the unicode maths characters, I think
I did the same but somehow letters like $xy$ just appear in roman format. I
have attached my file here.

Anyone knows why this happens?

-- 
``Life is not empty,
There is kindness, there is apple and there is faith

One day will come,
and to a mendicant I will endow a jasmine''

-- Sohrab Sepehri
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