assertEquals(1, frequency(list, "c"));
    assertEquals(0, frequency(list, "d"));
    assertEquals(0, frequency(list, 4.2));
    assertEquals(0, frequency(list, null));
  }

  public void testRemoveAll_collection() {
    List<String> list = newArrayList("a", "b", "c", "d", "e");
    assertTrue(Iterables.removeAll(list, newArrayList("b", "d", "f")));
    assertEquals(newArrayList("a", "c", "e"), list);