code: func Register(mut Mutation) {
  	gob.Register(mut)
  	t := reflect.TypeOf(mut)
  	registry[t.String()] = t
  }
docs: // Register allows |mut| to be played by the tumble backend. This should be
  // called at init() time once for every Mutation implementation.
  //
  // This will also gob.Register your mutation implementation.
  //
  // Example:
  //   Register((*MyMutationImpl)(nil))