code: def report_numbers2marc(self, key, value):
      
      def _get_mangled_source(source):
          if source == 'arXiv':
              return 'arXiv:reportnumber'
          return source
  
      source = _get_mangled_source(value.get('source'))
  
      if value.get('hidden'):
          return {
              '9': source,
              'z': value.get('value'),
          }
  
      return {
          '9': source,
          'a': value.get('value'),
      }
docs: Populate the ``037`` MARC field.