src/main/java/io/reactivex/Single.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    @SuppressWarnings({ "unchecked", "rawtypes" })
    public static <T> Single<T> merge(SingleSource<? extends SingleSource<? extends T>> source) {
        ObjectHelper.requireNonNull(source, "source is null");
        return RxJavaPlugins.onAssembly(new SingleFlatMap<SingleSource<? extends T>, T>(source, (Function)Functions.identity()));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    @SuppressWarnings({ "unchecked", "rawtypes" })
    public static <T> Single<T> merge(SingleSource<? extends SingleSource<? extends T>> source) {
        ObjectHelper.requireNonNull(source, "source is null");
        return RxJavaPlugins.onAssembly(new SingleFlatMap<SingleSource<? extends T>, T>(source, (Function)Functions.identity()));
    }
Flattens a {@code Single} that emits a {@code Single} into a single {@code Single} that emits the item
emitted by the nested {@code Single}, without any transformation.
<p>
<img width="640" height="412" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.merge.oo.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code merge} does not operate by default on a particular {@link Scheduler}.</dd>
<dd>The resulting {@code Single} emits the outer source's or the inner {@code SingleSource}'s {@code Throwable} as is.
Unlike the other {@code merge()} operators, this operator won't and can't produce a {@code CompositeException} because there is
only one possibility for the outer or the inner {@code SingleSource} to emit an {@code onError} signal.
Therefore, there is no need for a {@code mergeDelayError(SingleSource<SingleSource<T>>)} operator.
</dd>
</dl>

@param <T> the value type of the sources and the output
@param source
a {@code Single} that emits a {@code Single}
@return a {@code Single} that emits the item that is the result of flattening the {@code Single} emitted
by {@code source}
@see <a href="http://reactivex.io/documentation/operators/merge.html">ReactiveX operators documentation: Merge</a>