Using AVAudioRecorder to record the output of the mainMixerNode of an AVAudioEngine instance

I am trying to use the

AVAudioRecorder

method to record the output of the

mainMixerNode

of an

AVAudioEngine

instance and save it to an MPEG4ACC file. From what I have been reading the default input to

AVAudioRecorder

is the microphone. I have everything setup so I can record to a file but how can I change the

AVAudioRecorder

input to be the

mainMixerNode

output?