code: def test_parse_optional_grammar(self) -> None:
      grammar = """
          start: 'a' ['b']
          """
      rules = parse_string(grammar, GrammarParser)
      visitor = self.Visitor()
  
      visitor.visit(rules)
  
      # Grammar/Rule/Rhs/Alt/NamedItem/StringLeaf                       -> 6
      #                      NamedItem/Opt/Rhs/Alt/NamedItem/Stringleaf -> 6
  
      self.assertEqual(visitor.n_nodes, 12)
  
imports: import ast
  import difflib
  import io
  import textwrap
  import unittest
  from test import test_tools
  from typing import Dict, Any
  from tokenize import TokenInfo, NAME, NEWLINE, NUMBER, OP
label_desc: Use image node_id 5 for calling the TestGrammarVisitor obj's underlying member method code with example usage: obj.test_parse_optional_grammar() without return types