code: func (t *Topic) FromHex(hex string) error {
  	bytes, err := hexutil.Decode(hex)
  	if err != nil || len(bytes) != len(t) {
  		return NewErrorf(ErrInvalidValue, "Cannot decode topic")
  	}
  	copy(t[:], bytes)
  	return nil
  }
docs: // FromHex will parse a hex string into this Topic instance