@Test
public void testLazyFlatMapDoubling() {

  List<Integer> eval = mlazy(asList(1, 2, 3, 4, 5)).flatMap(MonadicsTest.<Integer>twice()).eval();
  assertArrayEquals(new Integer[]{1, 1, 2, 2, 3, 3, 4, 4, 5, 5}, eval.toArray(new Integer[]{}));
}