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