old_contents: import { combineReducers } from 'redux';
  import { routerReducer } from 'react-router-redux';
  
  const rootReducer = combineReducers({
    routing: routerReducer,
  });
  
  export default rootReducer;
  
new_contents: import { combineReducers } from 'redux';
  import { routerReducer } from 'react-router-redux';
  import header from './header';
  
  const rootReducer = combineReducers({
    header,
    routing: routerReducer,
  });
  
  export default rootReducer;
  
subject: add header reducer to root reducer