NavigableMap<String, Integer> map = create();
    map.put("a", 1);
    map.firstKey();
  }

  public void testFloorEntry() {
    create().floorEntry("a");
  }

  public void testFloorKey() {
    create().floorKey("a");
  }

  public void testHeadMap_k() {
    NavigableMap<String, Integer> map = create();
    SortedMap<String, Integer> headMap = map.headMap("a");