`xor` can contribute to `vc` assumption

[camoy]

I encountered the following strange behavior:

(define-symbolic x boolean?)
(define-symbolic y boolean?)

(xor (and (not x) (not y))
     (and (not x) y))

(vc)

Notice that the assumption component now has (|| (! (&& (! x) (! y))) (! (&& y (! x)))). Perhaps I have the wrong mental model of what's supposed to be happening here, but I definitely did not expect there to be anything added to the vc for what should be a pure error-free operation.