Creates an Observable from this stream that converts each element into zero or more events.
Each incoming event is converted to an Iterable of new events, and each of these new events are then sent by the returned Observable in order.
The returned Observable is a broadcast stream if this stream is. If a broadcast stream is listened to more than once, each subscription will individually call convert and expand the events.
@override Observable<S> expand<S>(Iterable<S> convert(T value)) => Observable<S>(_stream.expand(convert));