[NTG-context] index with more than one column?

How do I produce an index with more than one column? This produces one column only: \startchapter[title=\mytitle{Register}] \startcolumns[n=2] \placeregister[index] \stopcolumns Nor did \placeregister[index][n=2] or \placeindex[n=2], with or without the columns envirenment, produce the result wanted. The \startsimplecolumns[n=2] environment did produce two columns, but then played havoc with the textheight. dr. Hans van der Meer