hp prime programing with symbolic matrix

[kain_x_x]

Hi guys.
I'm trying to make a program with symbolic matrix.
My steps are the next:
first I make a matrix with zeros.
Then i tried to input in each element of the matrix an expresion in a for loop:
(z(i):=z(i)*(("X")-a(j)));

No mather if i put a "x" or an "X". I return from the program "incorrect argument"
If I put:
CAS (z(i):=z(i)*(("X")-a(j)));
then x will be replaced by zero because x haven't a value yet ( i don't want a value at all).

How can I make a matrix with symbolic elements?

Thanks in advance.