singleWhereOrNone method

Implementation

Option<E> singleWhereOrNone(bool Function(E element) test) {
  Option<E> result = const None();
  for (final element in this) {
    if (test(element)) {
      if (result.isSome()) {
        return const None();
      }
      result = Some(element);
    }
  }
  return result;
}