code: func ChannelID(env *cb.Envelope) (string, error) {
  	chdr, err := ChannelHeader(env)
  	if err != nil {
  		return "", errors.WithMessage(err, "error retrieving channel header")
  	}
  
  	return chdr.ChannelId, nil
  }
docs: // ChannelID returns the Channel ID for a given *cb.Envelope.