code: func (m *MACCommandBlock) Size() (int, error) {
  	var count int
  	for _, mc := range m.MACCommands {
  		b, err := mc.MarshalBinary()
  		if err != nil {
  			return 0, errors.Wrap(err, "marshal binary error")
  		}
  		count += len(b)
  	}
  	return count, nil
  }
docs: // Size returns the size (in bytes) of the mac-commands within this block.