[Zope] DTML Document ids

Can someone explain to me why in the following code the line starting with * outputs a null string and the line starting with ** outputs the correct id of the DTMLDocument object ? <dtml-var standard_html_header> <dtml-in expr="objectItems('DTML Document')" prefix="sequence"> *<dtml-var "sequence_item.id"><br/> **<dtml-var id><br/> </dtml-in> <dtml-var standard_html_footer> TIA __peppo