code: func (s *ShallowStorage) Shallow() ([]plumbing.Hash, error) {
  	shallow, err := s.temporal.Shallow()
  	if err != nil {
  		return nil, err
  	}
  
  	if len(shallow) != 0 {
  		return shallow, nil
  	}
  
  	return s.ShallowStorer.Shallow()
  }
docs: // Shallow honors the storer.ShallowStorer interface.