code: func (e *RecordEncoder) Tombstones(tstones []Stone, b []byte) []byte {
  	buf := encoding.Encbuf{B: b}
  	buf.PutByte(byte(RecordTombstones))
  
  	for _, s := range tstones {
  		for _, iv := range s.intervals {
  			buf.PutBE64(s.ref)
  			buf.PutVarint64(iv.Mint)
  			buf.PutVarint64(iv.Maxt)
  		}
  	}
  	return buf.Get()
  }
docs: // Tombstones appends the encoded tombstones to b and returns the resulting slice.