Goal:
  
  s : VSet.t
  s' : VSet.t
  s'' : VSet.t
  x : V.t
  hadd : VSetProp.Add x s s'
  hin : ~ VSet.In x s
  H : ~ VSet.In x s''
  ============================
   (~ VSet.In x (VSet.union s s''))


DEBUG:Going to execute:
unfold iff in *; fold any not; intros;
            VSetDecide.MSetDecideAuxiliary.abstract_elements;
            VSetDecide.MSetDecideAuxiliary.no_logical_interdep; decompose
            records; VSetDecide.MSetDecideAuxiliary.discard_nonMSet;
            unfold VSet.Empty, VSet.Subset, VSet.Equal in *; 
            intros; VSetDecide.MSetDecideAuxiliary.change_to_E_t;
            VSetDecide.MSetDecideAuxiliary.E_eq_to_Logic_eq; subst ++;
            VSetDecide.MSetDecideAuxiliary.Logic_eq_to_E_eq; pull not using
            MSet_decidability; unfold not in *