public final boolean containsSingle(
     @Nullable final KEYTYPE aKey, @Nullable final VALUETYPE aValue) {
   final COLLTYPE aCont = get(aKey);
   return aCont != null && aCont.contains(aValue);
 }
 @Nonnull
 public final EChange removeSingle(
     @Nullable final KEYTYPE aKey, @Nullable final VALUETYPE aValue) {
   final COLLTYPE aCont = get(aKey);
   return aCont == null ? EChange.UNCHANGED : EChange.valueOf(aCont.remove(aValue));
 }