static void Check(string input, string expected)
        {
            var tokens     = Tokenize(input);
            var inputTree  = CreateInMemoryModel(tokens);
            var calculator = new VariableCalculator();
            var underTest  = calculator.Simplify(inputTree);
            var asString   = new FormattingExpressionVisitor().Format(underTest);

            asString.Should().Be(expected);
        }
        static void Check(string input, string expected)
        {
            var tokens    = Tokenize(input);
            var inputTree = CreateInMemoryModel(tokens);
            var mover     = new AdditionAndSubtractionMover();
            var underTest = mover.Simplify(inputTree);
            var asString  = new FormattingExpressionVisitor().Format(underTest);

            asString.Should().Be(expected);
        }