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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def true_ces(subsystem, previous_state, next_state):
 """Set of all sets of elements that have true causes and true effects. .. note:: Since the true |CauseEffectStructure| is always about the full system, the background conditions don't matter and the subsystem should be conditioned on the current state. """

    network = subsystem.network
    nodes = subsystem.node_indices
    state = subsystem.state

    _events = events(network, previous_state, state, next_state, nodes)

    if not _events:
        log.info("Finished calculating, no echo events.")
        return None

    result = tuple([event.actual_cause for event in _events] +
                   [event.actual_effect for event in _events])
    log.info("Finished calculating true events.")
    log.debug("RESULT: \n%s", result)

    return result