src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    public final ParallelFlowable<T> filter(@NonNull Predicate<? super T> predicate) {
        ObjectHelper.requireNonNull(predicate, "predicate");
        return RxJavaPlugins.onAssembly(new ParallelFilter<T>(this, predicate));
    }
@CheckReturnValue
    public final ParallelFlowable<T> filter(@NonNull Predicate<? super T> predicate) {
        ObjectHelper.requireNonNull(predicate, "predicate");
        return RxJavaPlugins.onAssembly(new ParallelFilter<T>(this, predicate));
    }
Filters the source values on each 'rail'.
<p>
Note that the same predicate may be called from multiple threads concurrently.
@param predicate the function returning true to keep a value or false to drop a value
@return the new ParallelFlowable instance