Reports & Discoverer :: REP-1241 Circular Column Dependency Originating With Column?

I receive the error,

REP-1241 Circular column dependency originating with column 'cenvat_opening'

I have the following return statement for a column total_credit;

function total_creditFormula return Number is
begin
Return (return nvl(:cf_cenvat_closing ,0) - nvl(:cf_duty_on_goods_ed,0);
end;

Now i am using this total_creditFormula for the error formula column 'cenvat_opening'

function cenvat_openingFormula return Number is
f_num_opn number(14,2);
begin
SELECT NVL(Amount,0) INTO F_NUM_OPN

[code]...

here the formula column 'total_credit' has the following return statements;

RETURN Nvl(:cenvat_opening,0) + Nvl(:cenvat_credit_manu,0)

so how can i use this formula column? is there any other option to return the value in the same column.