You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: marc21.rb
    path of file: ./repos/archivesspace/backend/app/exporters/models
    
    the code of the file until where you have to start completion: # coding: utf-8
class MARCModel < ASpaceExport::ExportModel
  model_for :marc21

  include JSONModel

  def self.df_handler(name, tag, ind1, ind2, code)
    define_method(name) do |val|
      df(tag, ind1, ind2).with_sfs([code, val])
    end
    name.t
    
  def self.assemble_controlfield_string(obj)
    date = obj.dates[0] || {}
    string = obj['system_mtime'].scan(/\d{2}/)[1..3].join('')
    string += date['date_type'] == 'single' ? 's' : 'i'
    string += date['begin'] ? date['begin'][0..3] : "    "
    string += date['end'] ? date['end'][0..3] : "    "

    repo = obj['repository']['_resolved']

    if repo.has_key?('country') && !repo['country'].empty?
      string += (ISO_3166_TO_MARC[repo['country']] || "xx")
    else
      string += "xx"
    end