def _prefetcher_preprocess(self, batch):
        '''
        If frame_stack_preprocess is not set, each experience in the replay will be stored as a list of frames, as
        opposed to a single numpy array.  We must condense them into a single numpy array as that is what the
        aggregator expects.
        '''
        if not self.frame_stack_concatenate_on_env:
            batch = self.frame_stack_preprocess.preprocess_list(batch)
        batch = self.aggregator.aggregate(batch)
        return batch

        If frame_stack_preprocess is not set, each experience in the replay will be stored as a list of frames, as
        opposed to a single numpy array.  We must condense them into a single numpy array as that is what the
        aggregator expects.
        
surreal/learner/ddpg.py