IllegalArgumentException expected =
        assertThrows(IllegalArgumentException.class, () -> Stream.of(1, 2).collect(onlyElement()));
    assertThat(expected.getMessage()).contains("1, 2");
  }

  public void testOnlyElementMany() {
    IllegalArgumentException expected =
        assertThrows(
            IllegalArgumentException.class,
            () -> Stream.of(1, 2, 3, 4, 5, 6).collect(onlyElement()));