module ModuleLevelInheritableAttributes
  def self.included(base)
    base.extend(ClassMethods)
  end
  
  module ClassMethods
    def mattr_inheritable(*args)
      @mattr_inheritable_attrs ||= [:mattr_inheritable_attrs]
      @mattr_inheritable_attrs += args
      args.each do |arg|
        module_eval %(
          class << self; attr_accessor :#{arg} end
        )
      end
      @mattr_inheritable_attrs
    end

    def inherited(subclass)
      @mattr_inheritable_attrs.each do |inheritable_attribute|
        instance_var = "@#{inheritable_attribute}" 
        subclass.instance_variable_set(instance_var, instance_variable_get(instance_var))
      end
    end
  end
end