not operator in XQuery?

Dear XQuery expert:

Can some one explain to me how can I do logical 'not' in XQuery WHERE
clause now?
I take a look at http://www.w3.org/TR/xquery

The syntax seems allow me to do
let $a := <a>4</a>
where $a > 3 and $a < 5
return $a

I cannot find a logical "not" operator there. So it seems not allow me to do

let $a := <a>4</a>
where not ($a > 3 and $a < 5)
return $a

Is right? Why?
-- 
Frank Yung-Fong Tang
譚永鋒
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