DEMA indicator coding and trading system not working

DEMA (Double exponential moving average) formula is 

DEMA = ( 2 * EMA(n)) - EMA(EMA(n)) 

 where n= period

The first step to calculating DEMA is to calculate the EMA. Then, run an EMA calculation again, using the result of the first EMA calculation (EMA(n) as a function of the equation EMA(x) ). Finally, subtract the result from the product of 2 * EMA(n).

Dema trading strategy is buy when DEMA crosses over EMA from below and 

sell when DEMA crosses below EMA from above.

******************************

buy code :

SET A = EMA(CLOSE,9)
SET B = EMA(A,9)
SET C = (2 * A) - B
CROSSOVER(C,A)

**********************************

sell code :

SET A = EMA(CLOSE,9)
SET B = EMA(A,9)
SET C = (2 * A) - B
CROSSOVER(A,C)

 ********************************

the above code is not working 

the logic was correct but, can not be plotted, with transcript, SET C is not possible