[TYPO3] Extracting the second UID from the rootline

[Joerg Wagner - DigiLog multimedia]

Hi all!

I want to show only a subbranch of a menu. Which branch to show should 
depend on the page that is currently shown.

Example Menu:

    MenuitemA
        SubmenuitemA1
            SubmenuitemA1a
            SubmenuitemA1b
        SubmenuitemA2
    MenuitemB
        SubmenuitemB1
        ...
    MenuitemC
        SubmenuitemC1
        ...

If the current page is "SubmenuitemA1a"  (or ...A1b) then I want to show 
only the subbranch A1 of the menu:

    SubmenuitemA1
        SubmenuitemA1a
        SubmenuitemA1b
    SubmenuitemA2


I am close to a solution by using a TS structure like this:
  10 = HMENU
  10.special = directory
  10.special.value = XXX
but I would need a possibility to fetch the second uid from the rootline and 
feed it into XXX.  I just can't find a way to do that. Possibly stdwrap can 
do the trick. But how?!?

Any help is greatly appreciated!
THX.
--
Jörg Wagner
DigiLog multimedia