code: func (m SnapshotMeta) Compare(existing SnapshotMeta) SnapshotMetaRel {
  	switch {
  	case m.Lineage == "" || existing.Lineage == "":
  		return SnapshotLegacy
  	case m.Lineage != existing.Lineage:
  		return SnapshotUnrelated
  	case m.Serial > existing.Serial:
  		return SnapshotNewer
  	case m.Serial < existing.Serial:
  		return SnapshotOlder
  	default:
  		// both serials are equal, by elimination
  		return SnapshotEqual
  	}
  }
docs: // Compare determines the relationship, if any, between the given existing
  // SnapshotMeta and the potential "new" SnapshotMeta that is the receiver.