require 'set' # Deals with a collection of unordered values with no duplicates

Include this module to make your class taggable. The names of the
instance variable and the setup method are prefixed with "taggable_"
to reduce the risk of namespace collision. You must call
taggable_setup before you can use any of this module's methods.

module Taggable
  attr_accessor :tags
  def taggable_setup
    @tags = Set.new
  end
  def add_tag(tag)
    @tags << tag
  end
  def remove_tag(tag)
    @tags.delete(tag)
  end
end
module Taggable2
  def initialize(a,b,c)
  end
end
class TaggableString < String
  include Taggable
  def initialize(*args)
    super
    taggable_setup
  end
end
s = TaggableString.new('It was the best of times, it was the worst of times.')
s.add_tag 'dickens'
s.add_tag 'quotation'
s.tags

module Complaint 
  def gripe
    voice('In all my years I have never encountered such behavior...')
  end
  def faint_praise
    voice('I am pleased to notice some improvement, however slight...')
  end
  def voice(complaint_text)
    raise NotImplementedError,
    "#{self.class} included the Complaint module but didn't define voice!"
  end
end
class MyComplaint
  include Complaint
end
MyComplaint.new.gripe

NotImplementedError: MyComplaint included the Complaint module
but didn't define voice!

Error! (Exception?) Here's stdout:
NotImplementedError: MyComplaint included the Complaint module but didn't define voice!
	from (irb):37:in `voice'
	from (irb):31:in `gripe'
	from (irb):44

module Ayto
  def potato
    'Pohtayto'
  end
end
module Ahto
  def potato
    'Pohtahto'
  end
end
class Potato
  include Ayto
  include Ahto
end
Potato.new.potato