code: public final boolean onlySubsetModified (Set<String> fieldSet)
      {
          for (String field : _descripMap.keySet()) {
              if (isModified(field) && (!fieldSet.contains(field))) {
                  return false;
              }
          }
          return true;
      }
docs: Returns true only if the set of modified fields is a subset of the
  fields specified.