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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 _desy_bookkeeping2marc(self, key, value):
 """Populate the ``595_D`` MARC field. Also populates the ``035`` MARC field through side effects. """

    if 'identifier' not in value:
        return {
            'a': value.get('expert'),
            'd': value.get('date'),
            's': value.get('status'),
        }

    self.setdefault('035', []).append({
        '9': 'DESY',
        'z': value['identifier']
    })