symbolic results in CAS

[epp]

In CAS I can invoke Toolbox > Algebra > Factor as follows:

factor(x^4 - 1) --> (x-1)*(x+1)*(x^2+1)

So far so good. Now, if I store 2 into variable "x" and invoke factor again we have

factor(x^4 - 1) --> 15

In other words, if I have used a variable I can no longer obtain symbolic results with that variable. How do I mark a variable unused so it can be used for symbolic calculations?