<foo xmlns:petal="http://purl.org/petal/1.0/willBeReplaced"
     xmlns:petal-temp="http://purl.org/petal/1.0/"
     petal-temp:attributes="xmlns:petal string:http://purl.org/petal/1.0/">

  <bar baz="default" 
       petal:attributes="baz default"
       petal-temp:attributes="petal:attributes string:baz ${baz_value}" />

</foo>