<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 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])