@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));
 }
 @Test
 public void testToArray() {
   final String[] a = toArray(String.class, Collections.<String>list());
   assertEquals(0, a.length);
 }
 @Test
 public void testCons() {
   // compile check
   List<Object> x = Collections.<Object>cons("1", list(1, 3));
 }
 @Test
 public void mkString() {
   List<Integer> l = list(1, 2, 3);
   assertEquals("1-2-3", Collections.mkString(l, "-"));
 }