parse.go
func Must(op Op, err error) Op {
	if err != nil {
		panic(fmt.Errorf("unable to parse selector; %v", err.Error()))
	}

	return op
}
func Must(op Op, err error) Op {
	if err != nil {
		panic(fmt.Errorf("unable to parse selector; %v", err.Error()))
	}

	return op
}
// Must is a convenience method similar to template.Must