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

  public void testFrequency_list() {
    List<String> list = newArrayList("a", "b", "a", "c", "b", "a");
    assertEquals(3, frequency(list, "a"));
    assertEquals(2, frequency(list, "b"));
    assertEquals(1, frequency(list, "c"));