src/main/java/io/reactivex/parallel/ParallelFlowable.java
@CheckReturnValue
    @NonNull
    public final <U> ParallelFlowable<U> compose(@NonNull ParallelTransformer<T, U> composer) {
        return RxJavaPlugins.onAssembly(ObjectHelper.requireNonNull(composer, "composer is null").apply(this));
    }
@CheckReturnValue
    @NonNull
    public final <U> ParallelFlowable<U> compose(@NonNull ParallelTransformer<T, U> composer) {
        return RxJavaPlugins.onAssembly(ObjectHelper.requireNonNull(composer, "composer is null").apply(this));
    }
Allows composing operators, in assembly time, on top of this ParallelFlowable
and returns another ParallelFlowable with composed features.

@param <U> the output value type
@param composer the composer function from ParallelFlowable (this) to another ParallelFlowable
@return the ParallelFlowable returned by the function