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: singleton.rb
    path of file: ./repos/artichoke/artichoke-backend/src/extn/stdlib/vendor/gen
    
    the code of the file until where you have to start completion: # frozen_string_literal: false

# The Singleton module impleme
    
    def __init__(klass) # :nodoc:
      klass.instance_eval {
        @singleton__instance__ = nil
        @singleton__mutex__ = Thread::Mutex.new
      }
      def klass.instance # :nodoc:
        return @singleton__instance__ if @singleton__instance__
        @singleton__mutex__.synchronize {
          return @singleton__instance__ if @singleton__instance__
          @singleton__instance__ = new()
        }
        @singleton__instance__
      end
      klass
    end