Hello Helmut,
it is to support the Side Menu (nav:side) with an automatic Active Side Menu.
Please see: http://E-H-S.wikidot.com/incl:Active-Side-Menu and the Side Menu, where I started to implement it.
I have also thought of different Display Versions, with and without frames, depending on the colour schemes of pages and the available space (width) in the Side Menu.
The idea was to place this into a new page (to be included) in the Side Menu, to make editing easier for updates and maintenance, as some Side Menus already have a long and involved structure.