Goal:
    s' : VSet.t  t0 : VSet.t  a : V.t  H : VSet.In a s'
  ============================
   (VSet.In a (VSet.union t0 s'))


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