can't seem to get a nested iif to work

hi all,

i'm trying measure display except dimension payment method = direct debit or renewal payment method = direct debit i.e.

payment method    renewal payment method    result
direct debit            cash                                       null
direct debit            direct debit                            null
cash                        direct debit                           null
cash                        cash                                    1 (i'll replace 1 calculation on measure)
cash                        cheque                                1

its counting how many still have converted payment method direct debit.

i thought might this;
iif([payment method].[payment method].currentmember [payment method].[payment method].[direct debit],0,
(iif([renewal payment method].[renewal payment method].currentmember [renewal payment method].[renewal payment method].[direct debit],0, 1))

which hoping follow logic; if payment method = direct debit, measure = 0, else if renewal payment method = direct debit (and payment method <> direct debit previous if excluded it), measure =  0, else measure =  1.

any appreciated.

regards,
phil

after. 

iif([payment method].[payment method].currentmember [payment method].[payment method].[direct debit] or
 [renewal payment method].[renewal payment method].currentmember [renewal payment method].[renewal payment method].[direct debit]
 ,0, 1)
 
tried similar thing on adventure works

with set [selectedproducts] as

{[product].[product categories].[product].&[344],

[product].[product categories].[product].&[348],

[product].[product categories].[product].&[310],

[product].[product categories].[product].&[220]}

member [measures].[a] as

iif([product].[product categories].currentmember is [product].[product categories].[mountain-100 silver, 38]

,0, 1)

member [measures].[b] as

iif([date].[calendar year].currentmember is [date].[calendar year].[all periods]

,0, 1)

member [measures].[c] as

iif([product].[product categories].currentmember is [product].[product categories].[mountain-100 silver, 38]

or  [date].[calendar year].currentmember is [date].[calendar year].[all periods]

,0, 1)

select{[a], [b], [c]} on 0,

nonempty([selectedproducts]* [date].[calendar year].members) on 1

from [adventure works]

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