static void CheckEnumerable(IEnumerable <IExpression> input, int expectedPosition)
        {
            var underTest = new SimplificationEvaluator();
            var frozen    = input as IExpression[] ?? input.ToArray();
            var expected  = frozen.ElementAt(expectedPosition);

            underTest.FindSmallesExpressionInEnumerable(frozen).Should().Be(expected);
        }
        static void CheckCount(string input, int expected)
        {
            var tokens    = Tokenize(input);
            var inputTree = CreateInMemoryModel(tokens);
            var underTest = new SimplificationEvaluator();

            underTest.CountExpression(inputTree).Should().Be(expected);
        }