code: def test_nullable(self) -> None:
      grammar_source = """
          start: sign NUMBER
          sign: ['-' | '+']
          """
      grammar: Grammar = parse_string(grammar_source, GrammarParser)
      rules = grammar.rules
      nullables = compute_nullables(rules)
      self.assertNotIn(rules["start"], nullables)  # Not None!
      self.assertIn(rules["sign"], nullables)
  
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 17 for calling the TestPegen obj's underlying member method code with example usage: obj.test_nullable() without return types