title: How to provide initial state for a single multireducer sub-slice
body: <p>My project uses <a href="https://github.com/erikras/multireducer" rel="nofollow noreferrer">multireducer</a> to maintain many instances of the same reducer.
  My store is created using something like this:</p>
  
  <pre><code>const initialState = {};
  
  export default createStoreWithMiddleware(combineReducers({   
      foo: fooReducer,
      sorting: multireducer({
          sorterA: tableSorterReducer,
          sorterB: tableSorterReducer,
      }),
  }), initialState);
  </code></pre>
  
  <p>Now, I need to add an initial state for <code>sorterB</code>.
  If I add the following...</p>
  
  <pre><code>const initialState = {
      sorting: {
          sorterB: { key: 'foo', order: 'asc' }
      }
  };
  </code></pre>
  
  <p>... then <code>sorterA</code> is not created in the store at all, since it is not included in the initial value of <code>sorting</code>.</p>
  
  <p>So how can I specifiy an initial state for only part of a multireducer?</p>