old_contents: """Test the runtime.lexer module."""
  import unittest
  
  from runtime.lexer import TokenTuple, NUMBER, OPERATOR, WHITESPACE, IDENTIFIER, STRING, run
  
  class TestLexer(unittest.TestCase):
      """Test the lexer."""
      def test_simple(self):
          """A small collection of simple test cases."""
          test_cases = [
              ("3", [TokenTuple(value="3", kind=NUMBER)]),
              ("+", [TokenTuple(value="+", kind=OPERATOR)]),
              (" ", [TokenTuple(value=" ", kind=WHITESPACE)]),
              ("a", [TokenTuple(value="a", kind=IDENTIFIER)]),
              ("\"\"", [TokenTuple(value="\"\"", kind=STRING)]),
              ("\"a\" + \"4\"", [
                  TokenTuple(value="\"a\"", kind=STRING),
                  TokenTuple(value=" ", kind=WHITESPACE),
                  TokenTuple(value="+", kind=OPERATOR),
                  TokenTuple(value=" ", kind=WHITESPACE),
                  TokenTuple(value="\"4\"", kind=STRING),
              ]),
          ]
          for (case, expected) in test_cases:
              self.assertEqual(run(case), expected)
              
new_contents: """Test the runtime.lexer module."""
  import unittest
  
  from runtime.lexer import TokenTuple, NUMBER, OPERATOR, WHITESPACE, IDENTIFIER, STRING, run
  
  class TestLexer(unittest.TestCase):
      """Test the lexer."""
      def test_simple(self):
          """A small collection of simple test cases."""
          test_cases = [
              ("3", [TokenTuple(value="3", kind=NUMBER)]),
              ("+", [TokenTuple(value="+", kind=OPERATOR)]),
              (" ", [TokenTuple(value=" ", kind=WHITESPACE)]),
              ("a", [TokenTuple(value="a", kind=IDENTIFIER)]),
              ("\"\"", [TokenTuple(value="\"\"", kind=STRING)]),
              ("\"a\" + \"4\"", [
                  TokenTuple(value="\"a\"", kind=STRING),
                  TokenTuple(value=" ", kind=WHITESPACE),
                  TokenTuple(value="+", kind=OPERATOR),
                  TokenTuple(value=" ", kind=WHITESPACE),
                  TokenTuple(value="\"4\"", kind=STRING),
              ]),
              ("3.14", [TokenTuple(value="3.14", kind=NUMBER)]),
          ]
          for (case, expected) in test_cases:
              self.assertEqual(run(case), expected)
              
subject: Add tests for lexer