src/main/java/io/reactivex/plugins/RxJavaPlugins.java
public static void setOnCompletableAssembly(@Nullable Function<? super Completable, ? extends Completable> onCompletableAssembly) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onCompletableAssembly = onCompletableAssembly;
    }
public static void setOnCompletableAssembly(@Nullable Function<? super Completable, ? extends Completable> onCompletableAssembly) {
        if (lockdown) {
            throw new IllegalStateException("Plugins can't be changed anymore");
        }
        RxJavaPlugins.onCompletableAssembly = onCompletableAssembly;
    }
Sets the specific hook function.
@param onCompletableAssembly the hook function to set, null allowed