*** Lasts  7.00s -->
Input: \f -> f [[]] == [] && (f ["a"] == "a") && (f ["ab"] == "b") && (f ["abc"] == "c") && (f ["a","b"] == "ab") && (f ["a","bc"] == "ac") && (f ["ab","d"] == "bd") && (f ["ac","bd"] == "cd") && (f ["ac","bd","x"] == "cdx") && (f ["a","bd","x"] == "adx")