code: def record_add_subfield_into(rec, tag, subfield_code, value,
                               subfield_position=None,
                               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)
  
      if subfield_position is None:
          subfields.append((subfield_code, value))
      else:
          subfields.insert(subfield_position, (subfield_code, value))
docs: Add subfield into specified position.
  
      Specify the subfield by tag, field number and optionally by subfield
      position.