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


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