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

  public void testFrequency_set() {
    Set<String> set = newHashSet("a", "b", "c");
    assertEquals(1, frequency(set, "a"));
    assertEquals(1, frequency(set, "b"));
    assertEquals(1, frequency(set, "c"));