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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 _null_sia(subsystem, phi=0.0):
 """Return a |SystemIrreducibilityAnalysis| with zero |big_phi| and empty cause-effect structures. This is the analysis result for a reducible subsystem. """

    return SystemIrreducibilityAnalysis(subsystem=subsystem,
                                        cut_subsystem=subsystem,
                                        phi=phi,
                                        ces=_null_ces(subsystem),
                                        partitioned_ces=_null_ces(subsystem))