assertTrue(set.subSet("aaa", "zzz").isEmpty());
    assertThrows(IllegalArgumentException.class, () -> set.subSet("quick", "the"));
  }

  public void testExplicit_first() {
    SortedSet<String> set =
        ImmutableSortedSet.orderedBy(STRING_LENGTH)
            .add("in", "the", "quick", "jumped", "over", "a")
            .build();
    assertEquals("a", set.first());
  }