src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> flatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper) {
        return flatMapSingle(mapper, false);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> flatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper) {
        return flatMapSingle(mapper, false);
    }
Maps each element of the upstream Observable into SingleSources, subscribes to all of them
and merges their onSuccess values, in no particular order, into a single Observable sequence.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/flatMapSingle.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code flatMapSingle} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
@param <R> the result value type
@param mapper the function that received each source value and transforms them into SingleSources.
@return the new Observable instance