code: func newHelperMethodDoctype(ln *line, rslt *result, src *source, parent element, opts *Options) (*helperMethodDoctype, error) {
  	if len(ln.tokens) < 3 {
  		return nil, fmt.Errorf("doctype is not specified [file: %s][line: %d]", ln.fileName(), ln.no)
  	}
  
  	doctype := ln.tokens[2]
  
  	if _, ok := doctypes[doctype]; !ok {
  		return nil, fmt.Errorf("doctype is invalid [file: %s][line: %d][doctype: %s]", ln.fileName(), ln.no, doctype)
  	}
  
  	e := &helperMethodDoctype{
  		elementBase: newElementBase(ln, rslt, src, parent, opts),
  		doctype:     doctype,
  	}
  
  	return e, nil
  }
docs: // newHelperMethodDoctype creates and returns a helper method doctype.