assertSame(this.<String>of(), set.subSet("c", "d"));
  }

  public void testSingle_first() {
    SortedSet<String> set = of("e");
    assertEquals("e", set.first());
  }

  public void testSingle_last() {
    SortedSet<String> set = of("e");
    assertEquals("e", set.last());
  }

  @J2ktIncompatible
  @GwtIncompatible // SerializableTester
  public void testSingle_serialization() {