code: def _null_sia(subsystem, phi=0.0):
      
      return SystemIrreducibilityAnalysis(subsystem=subsystem,
                                          cut_subsystem=subsystem,
                                          phi=phi,
                                          ces=_null_ces(subsystem),
                                          partitioned_ces=_null_ces(subsystem))
docs: Return a |SystemIrreducibilityAnalysis| with zero |big_phi| and empty
      cause-effect structures.
  
      This is the analysis result for a reducible subsystem.