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


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