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:
autorewrite with set_eq_simpl in *;
            VSetDecide.MSetDecideAuxiliary.inst_MSet_hypotheses; autorewrite
            with set_simpl set_eq_simpl in *; push not in * using
            MSet_decidability; VSetDecide.MSetDecideAuxiliary.substMSet