[NTG-context] How to get attribute value from xml tag?

Hi, Struggling with XML in ConTeXt. I'have the following tag in my xml file: ... <invoice>   <travel>     <distance unit="km">20</distance>   </travel> ... </invoice> \startxmlsetups xml:invoice:travel   \xmlfirst{#1}{distance} \stopxmlsetups The \xmlfirst{#1}{distance} yields /20/, but how can the unit attribute value /km/ be accessed? Gerard --