code: 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
  }
docs: // childOf returns true if the line is a child of the element.