src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @BackpressureSupport(BackpressureKind.FULL)
    public static Completable merge(Publisher<? extends CompletableSource> sources, int maxConcurrency) {
        return merge0(sources, maxConcurrency, false);
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    @BackpressureSupport(BackpressureKind.FULL)
    public static Completable merge(Publisher<? extends CompletableSource> sources, int maxConcurrency) {
        return merge0(sources, maxConcurrency, false);
    }
Returns a Completable instance that keeps subscriptions to a limited number of sources at once and
completes only when all source Completables complete or one of them emits an error.
<p>
<img width="640" height="269" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.merge.pn.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The returned {@code Completable} honors the backpressure of the downstream consumer
and expects the other {@code Publisher} to honor it as well.</dd>
<dt><b>Scheduler:</b></dt>
<dd>{@code merge} does not operate by default on a particular {@link Scheduler}.</dd>
<dt><b>Error handling:</b></dt>
<dd>If any of the source {@code CompletableSource}s signal a {@code Throwable} via {@code onError}, the resulting
{@code Completable} terminates with that {@code Throwable} and all other source {@code CompletableSource}s are disposed.
If more than one {@code CompletableSource} signals an error, the resulting {@code Completable} may terminate with the
first one's error or, depending on the concurrency of the sources, may terminate with a
{@code CompositeException} containing two or more of the various error signals.
{@code Throwable}s that didn't make into the composite will be sent (individually) to the global error handler via
{@link RxJavaPlugins#onError(Throwable)} method as {@code UndeliverableException} errors. Similarly, {@code Throwable}s
signaled by source(s) after the returned {@code Completable} has been disposed or terminated with a
(composite) error will be sent to the same global error handler.
Use {@link #mergeDelayError(Publisher, int)} to merge sources and terminate only when all source {@code CompletableSource}s
have completed or failed with an error.
</dd>
</dl>
@param sources the iterable sequence of sources.
@param maxConcurrency the maximum number of concurrent subscriptions
@return the new Completable instance
@throws NullPointerException if sources is null
@throws IllegalArgumentException if maxConcurrency is less than 1
@see #mergeDelayError(Publisher, int)