How to apply a constraint to all instances within a relationship?

  Hi all,

there is the following product structure in Siebel 7.8:

Root
    Cash
    Base
        SKAC
            Cash

I have written a rule to make sure that
-  if you have 1 Cash selected on Root, then at least 1 Cash is also selected on SKAC.
- And Cash will be removed from SKAC when Cash is removed from Root.

The rule looks as follows:
When the quantity of "Cash" from RelCash > 0 the quantity of RelBase >> Base >> RelSKAC >> SKAC >> RelCash >> Cash > 0, otherwise the quantity of RelBase >> Base >> RelSKAC >> SKAC >> RelCash >>Cash = 0

This works, but not if there are more SKAC instances under Base. However this rule should be applied on every instance.