code: function combineReducers (reducers) {
    const databaseReducers = Object.keys(reducers)
      .map((collectionName) => {
        const reducer = reducers[ collectionName ]
        return createDatabaseReducer(reducer, collectionName)
      })
  
    return (database, event) => {
      const changesets = databaseReducers.map((reducer) => reducer(database, event))
      return combineChangesets(changesets)
    }
  }
docs: Will combine multiple collection reducers to one database reducer.
  Takes an object whose keys are collection names and whose values are
  reducer functions.
  
  The input reducers have the signature `(Collection, Event) => Changeset`,
  the resulting combined reducer is `(Database, Event) => Changeset`.
  
  @param {object} reducers   { [collectionName: string]: Function }
  @return {Function}