code: func (o Options) GetOne(key OptionCode) ([]byte, error) {
  	vv, err := o.Get(key)
  	if err != nil {
  		return nil, err
  	}
  
  	if len(vv) != 1 {
  		return nil, ErrInvalidPacket
  	}
  	return vv[0], nil
  }
docs: // GetOne attempts to retrieve the first and only value specified by an
  // OptionCode key. GetOne should only be used for OptionCode keys that must
  // have at most one value.
  //
  // GetOne works just like Get, but if there is more than one value for the
  // OptionCode key, ErrInvalidPacket will be returned.