code: public static ErrorMessageFactory shouldContainOnly(Object actual, Object expected, Object notFound, Object notExpected) {
      return new ShouldContainOnly(actual, expected, notFound, notExpected);
    }
docs: Creates a new </code>{@link ShouldContainOnly}</code>.
  @param actual the actual value in the failed assertion.
  @param expected values expected to be contained in {@code actual}.
  @param notFound values in {@code expected} not found in {@code actual}.
  @param notExpected values in {@code actual} that were not in {@code expected}.
  @return the created {@code ErrorMessageFactory}.