code: @CheckReturnValue
      @BackpressureSupport(BackpressureKind.UNBOUNDED_IN)
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Single<Boolean> all(Predicate<? super T> predicate) {
          ObjectHelper.requireNonNull(predicate, "predicate is null");
          return RxJavaPlugins.onAssembly(new FlowableAllSingle<T>(this, predicate));
      }
docs: Returns a Single that emits a Boolean that indicates whether all of the items emitted by the source
  Publisher satisfy a condition.
  <p>
  <img width="640" height="315" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/all.png" alt="">
  <dl>
  <dt><b>Backpressure:</b></dt>
  <dd>The operator honors backpressure from downstream and consumes the source {@code Publisher} in an unbounded
  manner (i.e., without applying backpressure).</dd>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code all} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param predicate
  a function that evaluates an item and returns a Boolean
  @return a Single that emits {@code true} if all items emitted by the source Publisher satisfy the
  predicate; otherwise, {@code false}
  @see <a href="http://reactivex.io/documentation/operators/all.html">ReactiveX operators documentation: All</a>