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


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