code: void perform(ContentHandler handler, SectionState state) {
      final ModeUsage modeUsage = getModeUsage();
      if (handler != null)
        state.addActiveHandler(handler, modeUsage);
      else
        state.addAttributeValidationModeUsage(modeUsage);
      state.addChildMode(modeUsage, handler);
    }
docs: Performs this action on the section state.
  
  @param handler ???
  @param state The section state.