code: func blockTransform(s string) []string {
  	// TODO(philips): use spec/types.Hash after export typ field
  	parts := strings.SplitN(s, "-", 2)
  	if len(parts) != 2 {
  		panic(fmt.Errorf("blockTransform should never receive non-hash, got %v", s))
  	}
  	return []string{
  		parts[0],
  		parts[1][0:2],
  	}
  }
docs: // blockTransform creates a path slice from the given string to use as a
  // directory prefix. The string must be in hash format:
  //    "sha256-abcdefgh"... -> []{"sha256", "ab"}
  // Right now it just copies the default of git which is a two byte prefix. We
  // will likely want to add re-sharding later.