Goal:
    x : VSet.elt  p : VSet.t
  ============================
   (~ VSet.In x p -> VSet.Equal (VSet.remove x (VSet.add x p)) p)


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