+
+(* Given a language l, the Kleene's star of l, denoted by l*, is the set of
+finite-length strings that can be generated by concatenating arbitrary strings of
+l. In other words, w belongs to l* is and only if there exists a list of strings
+w1,w2,...wk all belonging to l, such that l = w1w2...wk.
+We need to define the latter operations. The following flatten function takes in
+input a list of words and concatenates them together. *)