code: func (p *PartitionedMemoryIdx) Prune(oldest time.Time) ([]idx.Archive, error) {
  	// Prune each partition sequentially.
  	result := []idx.Archive{}
  	for _, m := range p.Partition {
  		found, err := m.Prune(oldest)
  		if err != nil {
  			return result, err
  		}
  		result = append(result, found...)
  	}
  	return result, nil
  }
docs: // Prune deletes all metrics that haven't been seen since the given timestamp.
  // It returns all Archives deleted and any error encountered.