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I assume that the test expression in a conditional expression cannot be an updating expression. This would be parallel to not allowing the operand expression in a typeswitch expression to be an updating expression. This is not explicitly stated in section 2.4.3, Conditional Expression.
Ron, Thanks. You are right. The working group will correct this oversight in the next version of the XQuery update specification. Regards, Don Chamberlin (for the Query Working Group)