Goal:
    G : t  s0 : VSet.t  s' : VSet.t  x : V.t  t0 : VSet.t
  ============================
   (VSet.Subset s' (VSet.union t0 s'))


DEBUG:Going to execute:
tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_Prop P) holds by
            auto  100 with MSet_Prop then fail else clear H