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


DEBUG:Going to execute:
repeat match goal with
                  | x:_ |- _ => subst x
                  end