Goal:
    x : V.t  s : VSet.t
  ============================
   (~ VSet.In x (VSet.remove x s))


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