Goal:
  
  s : VSet.t
  s'' : VSet.t
  x : V.t
  hin : VSet.In x s -> False
  H : VSet.In x (VSet.union s s'')
  ============================
   (VSet.In x 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;
            VSetDecide.MSetDecideAuxiliary.assert_decidability