func MustParse(xp string) XPathExec {
	ret, err := Parse(xp)
	if err != nil {
		panic(err)
	}
	return ret
}
func MustParse(xp string) XPathExec {
	ret, err := Parse(xp)
	if err != nil {
		panic(err)
	}
	return ret
}
//MustParse is like Parse, but panics instead of returning an error.