assertFalse(c.containsAll(of("a", "d")));
    assertTrue(c.containsAll(of("a", "c")));
    assertTrue(c.containsAll(of("a", "b", "c")));
  }

  public void testEquals_sameType() {
    Collection<String> c = of("a", "b", "c");
    assertTrue(c.equals(of("a", "b", "c")));
    assertFalse(c.equals(of("a", "b", "d")));
  }