@Override
  public Collection<Rule> getAll() {
    return List.copyOf(rulesByUuid.values());
  }
@Test
  public void getAll_returns_immutable_empty_collection_when_register_was_never_called() {
    Collection<Rule> all = underTest.getAll();
    assertThat(all).isEmpty();

    assertThatThrownBy(() -> all.add(SOME_RULE))
      .isInstanceOf(UnsupportedOperationException.class);
  }