library_item.rb

module LibraryItem
  module Initializer
    def initialize *args
      @@group_identifier ||= 0
      @identifier = @@group_identifier += 1
      super *args
    end
  end

  module ClassMethods
    def count
      class_variable_get :@@group_identifier
    end
  end

  def eql? obj
    self.identifier == obj.identifier
  end

  def == obj
    self.eql? obj
  end

  def hash
    identifier
  end

  def self.included(klass)
    klass.send :extend, ClassMethods
    klass.send :attr_accessor, :identifier
    klass.send :prepend, Initializer
  end
end

class Bobobo
  include LibraryItem

  attr_accessor :attr

  def initialize attr = 0
    @attr = attr
  end

end