[NTG-context] Index - 'sort as' parameter for secondary and tertiary index entries

Dear All, while there is a sorting parameter for the primary index entry: \index[primary]{>primary<} I cannot find any method how to define 'sort as' parameter for next two levels: \index{primary+>aaa<} \index{primary+bbb} I want to have >aaa< before bbb in my index. Any idea? Thanks, Jan