Symmetric difference is associative?

Is the symmetric difference associative? I'd like to be able to give a proof for it.

I.e. is the following true? , where A, B, and C are sets.

To me it seems this is the case, i.e. that the symmetric difference is associative.

Let \ . I.e. assume that there is an element a in that is not in

This implies that a must be in only one of A, B or C for this to be true.

But if a is in either A, B or C, but not in two or more; then by definition of the symmetric difference, it must also be in Which is a contradiction -- a cannot be in without also being in

But how can I give a proof for this?