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 *;
            match goal with
            | H:VSet.In ?x ?r -> False
              |- VSet.In ?x ?s -> False =>
                  contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body
            | H:VSet.In ?x ?r -> False
              |- ?x = ?y -> False =>
                  contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body
            | H:VSet.In ?x ?r -> False
              |- ?y = ?x -> False =>
                  contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body
            | H:?P -> False
              |- ?Q -> False =>
                  tryif prop (VSetDecide.MSetDecideAuxiliary.MSet_elt_Prop P)
                   holds by auto  100 with MSet_Prop then
                   (contradict H; VSetDecide.MSetDecideAuxiliary.fsetdec_body)
                    else VSetDecide.MSetDecideAuxiliary.fsetdec_body 
            | |- _ => VSetDecide.MSetDecideAuxiliary.fsetdec_body
            end