code: function createDatabaseReducer (collectionReducer, collectionName) {
    return (database, event) => {
      const collection = database.collections[ collectionName ]
  
      if (!collection) {
        throw new Error(`Collection '${collectionName}' not known by database instance.`)
      }
  
      return collectionReducer(collection, event)
    }
  }
docs: Takes a collection reducer and turns it into a database reducer by binding it
  to one of the database's collections.
  
  @param {Function} collectionReducer    (Collection, Event) => Changeset
  @param {string}   collectionName
  @return {Function}                     (Database, Event) => Changeset