Preselected items fd:multivaluefield

[Amon]

Best Users,
 
I wonder how i can multiple preselected items in a fd:multivaluefield widget?
 
I have the following definition 
 
<fd:multivaluefield id="theme" > 
<fd:label>Theme:</fd:label>
<fd:datatype base="string"/> 
<fd:selection-list src="cocoon:/theme.xml"/> 
</fd:multivaluefield> 
  
Any help is very welcome,
 
Thanks in advance,
Amon
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