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
           | H:_ |- _ => progress change eq with eq in H
           | |- _ => progress change eq with eq
           end