Goal:
    x : VSet.elt  p : VSet.t  H : ~ VSet.In x p  a : VSet.elt
  ============================
   (VSet.In a (VSet.remove x (VSet.add x p)) <-> VSet.In a p)


DEBUG:Going to execute:
progress change T with V.t in H;
            repeat
             match goal with
             | J:_ |- _ => progress change T with V.t in J
             | |- _ => progress change T with V.t
             end