Once we know that a list has two data constructors, we can use them in the function argument for pattern matching. Hence, we can use  for empty list matching, and we can use x:y: to match two elements followed by an empty list.
In the snippet 13, we used an empty list pattern for checking whether a list is empty or not.
In the snippet 14, we used x:y: to check whether the list has length 2 or not. This might not be a very good thing if we want to check the larger size. There, we might use the length function to get the size of the list. However, be aware of the fact that the length function is not a constant time function, but proportional to the size of the list.