public IterableSubject factKeys() {
    if (!(actual instanceof ErrorWithFacts)) {
      failWithActual(simpleFact("expected a failure thrown by Truth's failure API"));
      return ignoreCheck().that(ImmutableList.of());
    }
    ErrorWithFacts error = (ErrorWithFacts) actual;
    return check("factKeys()").that(getFactKeys(error));
  }
@Test
  public void factKeysNoValue() {
    assertThat(simpleFact("foo")).factKeys().containsExactly("foo");
  }