// method.
func (p *Parser) SOAResource() (SOAResource, error) {
	if !p.resHeaderValid || p.resHeaderType != TypeSOA {
		return SOAResource{}, ErrNotStarted
	}
	r, err := unpackSOAResource(p.msg, p.off)
	if err != nil {
		return SOAResource{}, err
	}
	p.off += int(p.resHeaderLength)
	p.resHeaderValid = false
	p.index++
	return r, nil
}