[XQuery] can function be defined without a prefix?

Is the following legal?  Should it be?

declare default function namespace "my-namespace";
declare function ten() { 10 }

I see nothing in the specification to prohibit this.
Just "Every function must be in a namespace--that is, every declared 
function name must (when expanded) have a non-empty namespace URI."

Is it worth adding a clarification like this:
   It follows that the name of the declared function must have a prefix
   unless there is a 'default function namespace' declaration.
-- 
	--Per Bothner
per@bothner.com   http://per.bothner.com/