public void testCopyOf_collection_oneElement() {
    Collection<String> c = MinimalCollection.of("a");
    Set<String> set = copyOf(c);
    assertEquals(singleton("a"), set);
  }

  public void testCopyOf_collection_oneElementRepeated() {
    Collection<String> c = MinimalCollection.of("a", "a", "a");
    Set<String> set = copyOf(c);
    assertEquals(singleton("a"), set);
  }

  public void testCopyOf_collection_general() {