code: func (p *Prefix) UnmarshalText(txt []byte) error {
  	_, n, err := net.ParseCIDR(string(txt))
  	if err != nil {
  		return err
  	}
  	copy(p.IP.To16(), n.IP.To16())
  	copy(p.Mask, n.Mask)
  	return nil
  }
docs: // UnmarshalText replaces p with txt.