src/main/java/io/reactivex/plugins/RxJavaPlugins.java
@SuppressWarnings("rawtypes")
    public static void setOnParallelAssembly(@Nullable Function<? super ParallelFlowable, ? extends ParallelFlowable> handler) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        onParallelAssembly = handler;
    }
@SuppressWarnings("rawtypes")
    public static void setOnParallelAssembly(@Nullable Function<? super ParallelFlowable, ? extends ParallelFlowable> handler) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        onParallelAssembly = handler;
    }
Sets the specific hook function.
<p>History: 2.0.6 - experimental; 2.1 - beta
@param handler the hook function to set, null allowed
@since 2.2