Goal:
    _z_ : VSet.t  x0 : V.t  x1 : V.t  x2 : V.t  a : V.t
  ============================
   (VSet.In a (VSet.add x0 (VSet.add x1 (VSet.add x2 _z_))) <->
    VSet.In a (VSet.add x1 (VSet.add x2 (VSet.add x0 _z_))))


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