code: @SuppressWarnings("unchecked")
      @CheckReturnValue
      @BackpressureSupport(BackpressureKind.FULL)
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Flowable<T> startWith(Publisher<? extends T> other) {
          ObjectHelper.requireNonNull(other, "other is null");
          return concatArray(other, this);
      }
docs: Returns a Flowable that emits the items in a specified {@link Publisher} before it begins to emit
  items emitted by the source Publisher.
  <p>
  <img width="640" height="315" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/startWith.o.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The operator honors backpressure from downstream. Both this and the {@code other} {@code Publisher}s
  are expected to honor backpressure as well. If any of then violates this rule, it <em>may</em> throw an
  {@code IllegalStateException} when the source {@code Publisher} completes.</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code startWith} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param other
  a Publisher that contains the items you want the modified Publisher to emit first
  @return a Flowable that emits the items in the specified {@link Publisher} and then emits the items
  emitted by the source Publisher
  @see <a href="http://reactivex.io/documentation/operators/startwith.html">ReactiveX operators documentation: StartWith</a>