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: current_bundle.rb
    path of file: ./repos/shoulda-matchers/spec/support/tests
    
    the code of the file until where you have to start completion: require 'bundler'
require 'appraisal'

module Tests
  class CurrentBundle
    AppraisalNotSpecified = Class.new(ArgumentError)

    include Singleton

    def assert_appraisal!
      unless app
    
    def assert_appraisal!
      unless appraisal_in_use?
        message = <<EOT


Please run tests starting with `appraisal <appraisal_name>`.
Possible appraisals are: #{available_appraisals}

EOT
        raise AppraisalNotSpecified, message
      end