Goal:
    x : V.t  y : V.t  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:
PTRDEBUGTACrepeat match goal with
                  | x:_ |- _ => subst x
                  end