old_contents: import { combineReducers } from 'redux';
  
  import athlete from './athlete';
  
  export default combineReducers({
    athlete,
  });
  
new_contents: import { combineReducers } from 'redux';
  
  import athlete from './athlete';
  import activities from './activities';
  
  export default combineReducers({
    athlete,
    activities,
  });
  
subject: update root reducer.