anchor.diff:        expect(evaluator.parse('1 + 2 + 3 + 4').value, closeTo(10, epsilon));
         expect(evaluator.parse('1 + 2 + 3 + 4 + 5').value, closeTo(15, epsilon));
       });
  +    test('add parse', () {
  +      expect(parser.parse('1 + 2 + 3').value, [[1, '+', 2], '+', 3]);
  +    });
       test('sub', () {
         expect(evaluator.parse('1 - 2').value, closeTo(-1, epsilon));
         expect(evaluator.parse('1.2 - 1.2').value, closeTo(0, epsilon));
  
positive.code:       expect(evaluator.parse('2 + 3 * 4').value, closeTo(14, epsilon));
        expect(evaluator.parse('6 / 3 + 4').value, closeTo(6, epsilon));
        expect(evaluator.parse('2 + 6 / 2').value, closeTo(5, epsilon));
      });
      test('postfix add', () {
        expect(evaluator.parse('0++').value, closeTo(1, epsilon));
  
negative.code:       expect(evaluator.parse('1 - 2 - 3 - 4').value, closeTo(-8, epsilon));
        expect(evaluator.parse('1 - 2 - 3 - 4 - 5').value, closeTo(-13, epsilon));
      });
      test('sub parse', () {
        expect(parser.parse('1 - 2 - 3').value, [[1, '-', 2], '-', 3]);
      });
      test('mul', () {
        expect(evaluator.parse('2 * 3').value, closeTo(6, epsilon));
        expect(evaluator.parse('2 * -4').value, closeTo(-8, epsilon));
  