src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Flowable<R> flatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper) {
        return flatMapSingle(mapper, false, Integer.MAX_VALUE);
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Flowable<R> flatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper) {
        return flatMapSingle(mapper, false, Integer.MAX_VALUE);
    }
Maps each element of the upstream Flowable into SingleSources, subscribes to all of them
and merges their onSuccess values, in no particular order, into a single Flowable sequence.
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator consumes the upstream in an unbounded manner.</dd>
<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 Flowable instance