How do I use selected/checked ?

I'm a moron and I can't figure this out.

I know that to be xhtml compliant it must be
     selected="selected"
     checked="checked"

that's fine.

what I can't figure out is how to structure that in petal.

     <input type="radio" name="global" value="us"  
tal:attributes="????"/>

I can figure out how to do:
     selected="anystring"

but I can't figure out how to only include the selected attribute on  
the condition that it should be shown

suggestions please?