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


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