func (l *line) childOf(parent element) (bool, error) {
	var ok bool
	var err error

	switch {
	case l.isEmpty():
		ok = true
	case parent.ContainPlainText():
		switch {
		case parent.Base().ln.indent < l.indent:
			ok = true
		}
	default:
		switch {
		case l.indent == parent.Base().ln.indent+1:
			ok = true
		case l.indent > parent.Base().ln.indent+1:
			err = fmt.Errorf("the indent is invalid [file: %s][line: %d]", l.fileName(), l.no)
		}
	}

	return ok, err
}
func (l *line) childOf(parent element) (bool, error) {
	var ok bool
	var err error

	switch {
	case l.isEmpty():
		ok = true
	case parent.ContainPlainText():
		switch {
		case parent.Base().ln.indent < l.indent:
			ok = true
		}
	default:
		switch {
		case l.indent == parent.Base().ln.indent+1:
			ok = true
		case l.indent > parent.Base().ln.indent+1:
			err = fmt.Errorf("the indent is invalid [file: %s][line: %d]", l.fileName(), l.no)
		}
	}

	return ok, err
}
// childOf returns true if the line is a child of the element.