code: @SuppressWarnings("unchecked")
  	@Deprecated
  	protected static <T> Mono<T> onLastAssembly(Mono<T> source) {
  		Function<Publisher, Publisher> hook = Hooks.onLastOperatorHook;
  		if(hook == null) {
  			return source;
  		}
  		return (Mono<T>)Objects.requireNonNull(hook.apply(source), "LastOperator hook returned null");
  	}
docs: To be used by custom operators: invokes assembly {@link Hooks} pointcut given a
  {@link Mono}, potentially returning a new {@link Mono}. This is for example useful
  to activate cross-cutting concerns at assembly time, eg. a generalized
  {@link #checkpoint()}.
  
  @param <T> the value type
  @param source the source to apply assembly hooks onto
  
  @return the source, potentially wrapped with assembly time cross-cutting behavior
  @deprecated use {@link Operators#onLastAssembly(CorePublisher)}