code: func (d *RecordDecoder) Tombstones(rec []byte, tstones []Stone) ([]Stone, error) {
  	dec := encoding.Decbuf{B: rec}
  
  	if RecordType(dec.Byte()) != RecordTombstones {
  		return nil, errors.New("invalid record type")
  	}
  	for dec.Len() > 0 && dec.Err() == nil {
  		tstones = append(tstones, Stone{
  			ref: dec.Be64(),
  			intervals: Intervals{
  				{Mint: dec.Varint64(), Maxt: dec.Varint64()},
  			},
  		})
  	}
  	if dec.Err() != nil {
  		return nil, dec.Err()
  	}
  	if len(dec.B) > 0 {
  		return nil, errors.Errorf("unexpected %d bytes left in entry", len(dec.B))
  	}
  	return tstones, nil
  }
docs: // Tombstones appends tombstones in rec to the given slice.