is xquery support of function declaration in assertions

Hi Guys,
Did you know how to declare a XQuery function in XQuery expression window in assertions?
I'd like to declare a custom function to process some strings:

declare function local:comp($x as xs:string) as xs:string
{
   let $a := (substring($x,1,5))
   return $a
}

it seems to be failed in xquery assertion. i am not sure if i can declare a custom function in xquery window in soap ui.
is there a way to reference or declare a custom function for assertions?

thanks,
Aaron