public void DefaultValues()
        {
            var sut = new NullExpression();

            Assert.AreNotEqual(0, sut.GetHashCode());
            Assert.AreNotEqual(1, sut.GetHashCode());
        }
        public void Equality()
        {
            var a = new NullExpression();
            var b = new NullExpression();

            Assert.AreEqual(a, b);
            Assert.AreEqual(a.GetHashCode(), b.GetHashCode());
        }