code: def add_systemnumber(self, source, recid=None):
          
          if not recid:
              recid = self.get_recid()
          if not self.hidden and recid:
              record_add_field(
                  self.record,
                  tag='035',
                  subfields=[('9', source), ('a', recid)]
              )
docs: Add 035 number from 001 recid with given source.