public void Push_ValidArg_AddTheObjectToTheStact()
        {
            var stact = new Stact <string>();

            stact.Push("a");
            Assert.That(stact.Count, Is.EqualTo(1));
        }
        public void Push_ArgIsNull_ThrowArgumentNullException()
        {
            var stact = new Stact <string>();

            Assert.That(() => stact.Push(null), Throws.Exception.TypeOf <ArgumentNullException>());
        }