code: def test_multiple_nullable_rules(self) -> None:
      grammar = """
          start: sign thing other another $
          sign: ['-']
          thing: ['+']
          other: '*'
          another: '/'
          """
      self.assertEqual(
          self.calculate_first_sets(grammar),
          {
              "sign": {"", "'-'"},
              "thing": {"'+'", ""},
              "start": {"'+'", "'-'", "'*'"},
              "other": {"'*'"},
              "another": {"'/'"},
          },
      )
  
imports: import unittest
  from test import test_tools
  from typing import Dict, Set
label_desc: Use image node_id 22 for calling the TestFirstSets obj's underlying member method code with example usage: obj.test_multiple_nullable_rules() without return types