public void Print_ShouldPrintExpression()
        {
            var a = new Lit(7);
            var b = new Lit(8);
            var sub = new SubInt(a, b);

            Assert.That(sub.Print(), Is.EqualTo("7-8"));
        }
        public void Eval_ShouldAddTwoInteger()
        {
            var a = new Lit(7);
            var b = new Lit(8);
            var sub = new SubInt(a, b);

            Assert.That(sub.Eval(), Is.EqualTo(-1));
        }