Validates set of user-supplied target emotions.

def _check_emotion_set_is_supported(self):
        """
        Validates set of user-supplied target emotions.
        """
        supported_emotion_subsets = [
            set(['anger', 'fear', 'surprise', 'calm']),
            set(['happiness', 'disgust', 'surprise']),
            set(['anger', 'fear', 'surprise']),
            set(['anger', 'fear', 'calm']),
            set(['anger', 'happiness', 'calm']),
            set(['anger', 'fear', 'disgust']),
            set(['calm', 'disgust', 'surprise']),
            set(['sadness', 'disgust', 'surprise']),
            set(['anger', 'happiness'])
        ]
        if not set(self.target_emotions) in supported_emotion_subsets:
            error_string = 'Target emotions must be a supported subset. '
            error_string += 'Choose from one of the following emotion subset: \n'
            possible_subset_string = ''
            for emotion_set in supported_emotion_subsets:
                possible_subset_string += ', '.join(emotion_set)
                possible_subset_string += '\n'
            error_string += possible_subset_string
            raise ValueError(error_string)