Goal:
    x : VSet.elt  y : VSet.elt  p : VSet.t  H : x <> y
  ============================
   (VSet.Equal (VSet.add x (VSet.remove y p)) (VSet.remove y (VSet.add x p)))


DEBUG:Going to execute:
tryif change T with V.t in H then fail else
            tryif change T with VSet.t in H then fail else clear H