src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final Flowable<T> sorted(@NonNull Comparator<? super T> comparator) {
        return sorted(comparator, 16);
    }
@CheckReturnValue
    @NonNull
    public final Flowable<T> sorted(@NonNull Comparator<? super T> comparator) {
        return sorted(comparator, 16);
    }
Sorts the 'rails' of this ParallelFlowable and returns a Publisher that sequentially
picks the smallest next value from the rails.
<p>
This operator requires a finite source ParallelFlowable.

@param comparator the comparator to use
@return the new Flowable instance