code: public StringSubject factValue(String key, int index) {
      checkArgument(index >= 0, "index must be nonnegative: %s", index);
      return doFactValue(key, index);
    }
docs: Returns a subject for the value of the {@code index}-th instance of the fact with the given
  name. Most Truth failures do not contain multiple facts with the same key, so most tests should
  use {@linkplain #factValue(String) the other overload} of {@code factValue}.