code: def record_modify_subfield(rec, tag, subfield_code, value, subfield_position,
                             field_position_global=None,
                             field_position_local=None):
      
      subfields = record_get_subfields(
          rec, tag,
          field_position_global=field_position_global,
          field_position_local=field_position_local)
  
      try:
          subfields[subfield_position] = (subfield_code, value)
      except IndexError:
          raise InvenioBibRecordFieldError(
              "There is no subfield with position '%d'." % subfield_position)
docs: Modify subfield at specified position.
  
      Specify the subfield by tag, field number and subfield position.