How to use the nlu.messages.SystemMessage function in nlu

def main():
    logger = logging.getLogger()

    confidence_threshold = .5
    nlu = NaturalLanguageUnderstander(confidence_threshold, logger)

    # Register the NLU messages we want
    nlu.register_message(YesNoMessage)
    nlu.register_message(SearchMessage)
    nlu.register_message(SystemMessage)

    fil = open('INGREDIENTS.txt', 'r')

    # Skip comments and empty lines
    input_messages = (l.strip() for l in fil if not re.search('^\s*($|#)', l))
    for line in islice(input_messages, 0, LIMIT):
        messages = nlu.parse_input(line)
        print line
        for message in messages:
            print message.__class__
            print pprint.pformat(message.frame)
        print

# use the NLG yet; it uses a special 'echo' message that passes strings
        # through the NLG to the user.

        # If we didn't understand the user:
        if not parsed_input:
            return ContentPlanMessage("unknown")

        # For now, take first (highest confidence) message.
        # If the user is  searching for recipes:
        if isinstance(parsed_input[0], SearchMessage):
            return self._handle_search_message(parsed_input[0])

        # Handle system messages like 'restart' and 'exit'
        elif any(isinstance(i, SystemMessage) for i in parsed_input):
            for message in parsed_input:
                if isinstance(message, SystemMessage):
                    sys_message = message
                    break
            if sys_message.frame['action'] == 'restart':
                self._go_to_start_state()
                return ContentPlanMessage("echo",
                    message="Okay, let's start over.")
            elif sys_message.frame['action'] == 'exit':
                return ContentPlanMessage("echo",
                    message="TODO: EXIT HERE")

        # If the user answers 'yes', show them the recipes:
        elif isinstance(parsed_input[0], YesNoMessage) and self.search_results:
            if parsed_input[0].getDecision():
                return ContentPlanMessage("show_recipe",
                                          recipe=self.search_results[0])
            else:

# users to search for recipes and choose whether or not they want to
        # display the titles of the recipes in the search results.  It doesn't
        # use the NLG yet; it uses a special 'echo' message that passes strings
        # through the NLG to the user.

        # If we didn't understand the user:
        if not parsed_input:
            return ContentPlanMessage("unknown")

        # For now, take first (highest confidence) message.
        # If the user is  searching for recipes:
        if isinstance(parsed_input[0], SearchMessage):
            return self._handle_search_message(parsed_input[0])

        # Handle system messages like 'restart' and 'exit'
        elif any(isinstance(i, SystemMessage) for i in parsed_input):
            for message in parsed_input:
                if isinstance(message, SystemMessage):
                    sys_message = message
                    break
            if sys_message.frame['action'] == 'restart':
                self._go_to_start_state()
                return ContentPlanMessage("echo",
                    message="Okay, let's start over.")
            elif sys_message.frame['action'] == 'exit':
                return ContentPlanMessage("echo",
                    message="TODO: EXIT HERE")

        # If the user answers 'yes', show them the recipes:
        elif isinstance(parsed_input[0], YesNoMessage) and self.search_results:
            if parsed_input[0].getDecision():
                return ContentPlanMessage("show_recipe",