assertThat(set.headSet("g")).contains("e");
    assertSame(this.<String>of(), set.headSet("c"));
    assertSame(this.<String>of(), set.headSet("e"));
  }

  public void testSingle_tailSet() {
    SortedSet<String> set = of("e");
    assertTrue(set.tailSet("c") instanceof ImmutableSortedSet);
    assertThat(set.tailSet("c")).contains("e");
    assertThat(set.tailSet("e")).contains("e");