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: backend_services.rb
    path of file: ./repos/vets-api/lib
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

class Backend
    
  def self.all
    constants.map do |const|
      val = const_get const
      next unless val.is_a?(String)

      val
    end.compact
  end

  GI_BILL_STATUS = 'gibs'

  FACILITIES = 'facilities'
  HCA = 'hca'
  EDUCATION_BENEFITS = 'edu-benefits'
  EVSS_CLAIMS = 'evss-claims'
  LIGHTHOUSE = 'lighthouse'
  FORM526 = 'form526'
  ADD_PERSON_PROXY = 'add-person'
  APPEALS_STATUS = 'appeals-status'
  USER_PROFILE = 'user-profile'
  ID_CARD = 'id-card'
  IDENTITY_PROOFED = 'identity-proofed'
  VET360 = 'vet360'

  # MHV services
  RX = 'rx'
  MESSAGING = 'messaging'
  MEDICAL_RECORDS = 'medical-records'
  HEALTH_RECORDS = 'health-records'
  MHV_AC = 'mhv-accounts'

  # Core Form Features
  SAVE_IN_PROGRESS = 'form-save-in-progress'
  FORM_PREFILL = 'form-prefill'
end