code: public LocalProperties addUniqueFields(FieldSet uniqueFields) {
  		LocalProperties copy = clone();
  		
  		if (copy.uniqueFields == null) {
  			copy.uniqueFields = new HashSet<FieldSet>();
  		}
  		copy.uniqueFields.add(uniqueFields);
  		return copy;
  	}
docs: Adds a combination of fields that are unique in these data properties.
  
  @param uniqueFields The fields that are unique in these data properties.