@Override
  public int hashCode() {
    return Objects.hash(start, end);
  }
@Test
  public void hashcode_is_based__on_start_and_end() {
    assertThat(new TextBlock(15, 15)).hasSameHashCodeAs(new TextBlock(15, 15));
    assertThat(new TextBlock(15, 300)).hasSameHashCodeAs(new TextBlock(15, 300));
    assertThat(new TextBlock(15, 300).hashCode()).isNotEqualTo(new TextBlock(15, 15).hashCode());
  }