code: func HexID(in string) (NodeID, error) {
  	var id NodeID
  	b, err := hex.DecodeString(strings.TrimPrefix(in, "0x"))
  	if err != nil {
  		return id, err
  	} else if len(b) != len(id) {
  		return id, fmt.Errorf("wrong length, want %d hex chars", len(id)*2)
  	}
  	copy(id[:], b)
  	return id, nil
  }
docs: // HexID converts a hex string to a NodeID.
  // The string may be prefixed with 0x.