App.js

// ...
// initial state
const initialState = {
        left: CounterPair.reducer(),
        right: CounterPair.reducer(),
        count: 0
    }

// listen to any action of subcomponents
const appReducer = createReducer(
        initialState,
        {
            [Counter.INCREMENT]: state => updateProperty(state, 'count', state.count + 1)
        }
    )

// actual reducer
export const reducer = (state = initialState, action) => {
    // child reducers
    state = updateProperty(state, 'left', CounterPair.reducer(state.left, actionFor(LEFT, action)))
    state = updateProperty(state, 'right', CounterPair.reducer(state.right, actionFor(RIGHT, action)))

    // listen to any children event
    return appReducer(state, unwrap(action))
}
// ...