code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final Maybe<T> filter(Predicate<? super T> predicate) {
          ObjectHelper.requireNonNull(predicate, "predicate is null");
          return RxJavaPlugins.onAssembly(new MaybeFilter<T>(this, predicate));
      }
docs: Filters the success item of the Maybe via a predicate function and emitting it if the predicate
  returns true, completing otherwise.
  <p>
  <img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/filter.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code filter} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  
  @param predicate
  a function that evaluates the item emitted by the source Maybe, returning {@code true}
  if it passes the filter
  @return a Maybe that emit the item emitted by the source Maybe that the filter
  evaluates as {@code true}
  @see <a href="http://reactivex.io/documentation/operators/filter.html">ReactiveX operators documentation: Filter</a>