public void should_peek_fifo()
        {
            // Arrange
            var queue = new QueueWithStacks <int>();

            queue.Add(42);
            queue.Add(43);

            // Assert
            queue.Peek().Should().Be(42);
        }
        public void should_not_be_empty_after_a_value_is_added()
        {
            // Arrange
            var queue = new QueueWithStacks <int>();

            queue.Add(42);

            // Assert
            queue.IsEmpty().Should().BeFalse();
        }
        public void should_be_empty_after_last_value_is_removed()
        {
            // Arrange
            var queue = new QueueWithStacks <int>();

            queue.Add(42);

            // Act
            queue.Remove();

            // Assert
            queue.IsEmpty().Should().BeTrue();
        }