assertThat(map.get("four")).isNull();
    strings.add("four");
    assertEquals(ImmutableSortedMap.of("one", 3, "two", 3, "three", 5, "four", 4), map);
    assertEquals(Integer.valueOf(4), map.get("four"));
    SortedMap<String, Integer> headMap = map.headMap("two");
    assertEquals(ImmutableSortedMap.of("four", 4, "one", 3, "three", 5), headMap);
    strings.add("five");

    assertThat(map.get("four")).isNull();
    strings.add("four");
    assertEquals(ImmutableSortedMap.of("one", 3, "two", 3, "three", 5, "four", 4), map);
    assertEquals(Integer.valueOf(4), map.get("four"));
    SortedMap<String, Integer> headMap = map.headMap("two");
    assertEquals(ImmutableSortedMap.of("four", 4, "one", 3, "three", 5), headMap);
    strings.add("five");

          ClosingFuture<V1> future1, ClosingFuture<V2> future2, ClosingFuture<V3> future3) {
    return new Combiner3<>(future1, future2, future3);
  }

  /**
   * Starts specifying how to combine four {@link ClosingFuture}s into a single pipeline, assuming
   * they all succeed. If any fail, the resulting pipeline will fail.
   *
   * <p>Calling this method allows you to use lambdas or method references typed with the types of

    Iterator<String> iterator = asList("one", "two", "three", "four", "five").iterator();
    IllegalArgumentException expected =
        assertThrows(IllegalArgumentException.class, () -> getOnlyElement(iterator));
    assertThat(expected)
        .hasMessageThat()
        .isEqualTo("expected one element but was: <one, two, three, four, five>");
  }

  public void testGetOnlyElement_noDefault_moreThanFiveElements() {

    checkNotNull(a);
    checkNotNull(b);
    checkNotNull(c);
    return concat(consumingForArray(a, b, c));
  }

  /**
   * Combines four iterators into a single iterator. The returned iterator iterates across the
   * elements in {@code a}, followed by the elements in {@code b}, followed by the elements in

    // em space
    assertThat(parse("http://h/\u2003").encodedPath).isEqualTo("/%E2%80%83")
    // three-per-em space
    assertThat(parse("http://h/\u2004").encodedPath).isEqualTo("/%E2%80%84")
    // four-per-em space
    assertThat(parse("http://h/\u2005").encodedPath).isEqualTo("/%E2%80%85")
    // six-per-em space
    assertThat(parse("http://h/\u2006").encodedPath).isEqualTo("/%E2%80%86")
    // figure space

    abstract Equivalence<Object> defaultEquivalence();
  }

  /**
   * A helper object for operating on {@link InternalEntry} instances in a type-safe and efficient
   * manner.
   *
   * <p>For each of the four combinations of strong/weak key and strong/weak value, there are
   * corresponding {@link InternalEntry}, {@link Segment}, and {@link InternalEntryHelper}
   * implementations.
   *