Combines a sequence of values by repeatedly applying
Similar to Iterable.reduce, this function maintains a value,
starting with the first element of this stream
and updated for each further element of this stream.
For each element after the first,
the value is updated to the result of calling
with the previous value and the element.
When this stream is done, the returned future is completed with the value at that time.
If this stream is empty, the returned future is completed with
If this stream emits an error, or the call to
the returned future is completed with that error,
and processing is stopped.
@override AsObservableFuture<T> reduce(T combine(T previous, T element)) => AsObservableFuture<T>(_stream.reduce(combine));