2_b_2: Add solution

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+-- Write a function lastButOne, that returns the element before the last.
+
+-- This function is written in a bit cumbersome way. This is because we have
+-- avoided using functions introduced in later chapters of the book (e.g.,
+-- length, pattern matching). The myDrop function has been used as an example.
+--
+-- This function expects a list of at least two elements so the element before
+-- the last one always exists.
+--
+-- Checking the length of the list deserves some explanation. A call to tail
+-- discards the first element from the list. It is like subtracting 1 from the
+-- length of the list. Let's have a list of length N. If a list of length N - 2
+-- (two calls to tail) is not empty, then N is greater or equal to 3. If it is
+-- empty, then N has to be 2. It should not be less than 2 because that is a
+-- minimum length of the list expected.
+
+lastButOne :: [a] -> a
+lastButOne xs = if null (tail (tail xs)) -- Check if xs is 2 elements long
+                then head xs
+                else lastButOne (tail xs)