docs: def map_system_entities(engine, metadata, reset):
      """
      Maps all system entities.
      """
      # Map the user message system entity.
      msg_tbl = Table('_user_messages', metadata,
                      Column('guid', String, nullable=False, primary_key=True),
                      Column('text', String, nullable=False),
                      Column('time_stamp', DateTime(timezone=True),
                             nullable=False, default=sa_func.now()),
                      )
      mapper(UserMessage, msg_tbl, id_attribute='guid')
      if reset:
          metadata.drop_all(bind=engine, tables=[msg_tbl])
      metadata.create_all(bind=engine, tables=[msg_tbl])