@Test
public void testConcat() {

  final List<List<Integer>> l = list(list(1), list(2, 3), Collections.<Integer>nil(), list(9, 2, 1));
  final List<Integer> c = concat(l);
  assertArrayEquals(array(1, 2, 3, 9, 2, 1), toArray(Integer.class, c));
}