code: public <R> Stream<R> invokeAnd(Function<C, R> invoker) {
  			Function<C, InvocationResult<R>> mapper = (callbackInstance) -> invoke(
  					callbackInstance, () -> invoker.apply(callbackInstance));
  			return this.callbackInstances.stream().map(mapper)
  					.filter(InvocationResult::hasResult).map(InvocationResult::get);
  		}
docs: Invoke the callback instances where the callback method returns a result.
  @param invoker the invoker used to invoke the callback
  @param <R> the result type
  @return the results of the invocation (may be an empty stream if no callbacks
  could be called)