I need some advice on preventing infinite recursion in the following example:

I have a product dimension with three levels

All Product
Product Category
Product Group

A calculated measure Commission Per Trade is valid only for a) All level, b)
Product Category = Commission, c) All descendants of Commission product
category. Otherwise it is NULL.

I have a formula like this:

MEMBER [Measures].[rpt] as
'iif ([Product].CurrentMember = [Product].[All Product].[Commission] OR
(IsAncestor([Product].[All Product].[Commission],
[Product].CurrentMember))
AND [Measures].[Trade Count] > 0,
[Measures].[Revenue] / [Measures].[Trade Count], null)'

I haven't added the test of All Product, the formula at this point is going
into infinite recursion when selected for product dimension. Any test on
product current member seems to cause the recursion and I think I understand
it. I need some ideas in fixing the problem.

Thanks in advance
Murthy