Inconsistent expansion of {} inside strings ...

I ran into this today and it surprised me.  I'm sure this is "as 
documented" but was wondering if anyone could provide a rationale ?

{} expansion occurs within strings in attributes but not in "plain strings"

example:

let $a := "foo"
return <elem attr="xxx{$a}">{concat("xxx{$a}","bar")}</elem>


Result:
<elem attr="xxxfoo">xxx{$a}bar</elem>




-- 
-------------------------
David A. Lee

http://www.calldei.com
http://www.xmlsh.org