# No usage of private_class_method here
module Singleton
  def self.included(base)
    base.send(:extend, ClassMethods)
  end

  module ClassMethods

    def instance
      singleton_class.send(:private, :new) unless is_private_method?(:new)
      @instance ||= new
    end

    private
    def is_private_method?(meth_name)
      self.private_methods.include?(meth_name)
    end
  end

end
class SingletonIvarWoPrivateClassMethods
  include Singleton

  def foo
    12
  end
end

singletion = SingletonIvarWoPrivateClassMethods.instance
p singletion.object_id #=> 70174947355600

singleton_again = SingletonIvarWoPrivateClassMethods.instance
p singleton_again.object_id #=> 70174947355600

SingletonIvarWoPrivateClassMethods.new
#=> singleton_wo_private_class_method.rb:35:in `<main>': private method `new'
#   called for SingletonIvarWoPrivateClassMethods:Class (NoMethodError)