*
   * @since 20.0 (varargs overload since 2.0)
   */
  public static void checkArgument(
      boolean expression, String errorMessageTemplate, char p1, char p2) {
    if (!expression) {
      throw new IllegalArgumentException(lenientFormat(errorMessageTemplate, p1, p2));
    }
  }

  /**

    Iterator<String> iterator = Collections.singletonList("foo").iterator();
    assertEquals("foo", Iterators.getOnlyElement(iterator, "bar"));
  }

  public void testGetOnlyElement_withDefault_empty() {
    Iterator<String> iterator = Iterators.emptyIterator();
    assertEquals("bar", Iterators.getOnlyElement(iterator, "bar"));
  }

  public void testGetOnlyElement_withDefault_empty_null() {

    set.add(new Derived("foo"));
    set.add(new Derived("bar"));
    assertThat(set).containsExactly(new Derived("bar"), new Derived("foo")).inOrder();
  }

  public void testNewTreeSetEmptyNonGeneric() {
    TreeSet<LegacyComparable> set = Sets.newTreeSet();
    assertTrue(set.isEmpty());
    set.add(new LegacyComparable("foo"));
    set.add(new LegacyComparable("bar"));
    assertThat(set)

    TreeMap<Derived, Integer> map = Maps.newTreeMap();
    assertEquals(Collections.emptyMap(), map);
    map.put(new Derived("foo"), 1);
    map.put(new Derived("bar"), 2);
    assertThat(map.keySet()).containsExactly(new Derived("bar"), new Derived("foo")).inOrder();
    assertThat(map.values()).containsExactly(2, 1).inOrder();
    assertNull(map.comparator());
  }

  public void testTreeMapNonGeneric() {

    TreeMap<Derived, Integer> map = Maps.newTreeMap();
    assertEquals(Collections.emptyMap(), map);
    map.put(new Derived("foo"), 1);
    map.put(new Derived("bar"), 2);
    assertThat(map.keySet()).containsExactly(new Derived("bar"), new Derived("foo")).inOrder();
    assertThat(map.values()).containsExactly(2, 1).inOrder();
    assertNull(map.comparator());
  }

  public void testTreeMapNonGeneric() {

    Iterator<String> iterator = Collections.singletonList("foo").iterator();
    assertEquals("foo", Iterators.getOnlyElement(iterator, "bar"));
  }

  public void testGetOnlyElement_withDefault_empty() {
    Iterator<String> iterator = Iterators.emptyIterator();
    assertEquals("bar", Iterators.getOnlyElement(iterator, "bar"));
  }

  public void testGetOnlyElement_withDefault_empty_null() {

   *         .collect(
   *             flatteningToMultimap(
   *                  str -> str.charAt(0),
   *                  str -> str.substring(1).chars().mapToObj(c -> (char) c),
   *                  MultimapBuilder.linkedHashKeys().arrayListValues()::build));
   *
   * // is equivalent to
   *
   * static final ListMultimap<Character, Character> FIRST_LETTER_MULTIMAP;
   *