From http://robots.thoughtbot.com/evaluating-alternative-decorator-implementations-in

PORO_decorator.rb

class Coffee
  def cost
    2
  end

  def origin
    "Colombia"
  end
end

class Milk
  def initialize(component)
    @component = component
  end

  def cost
    @component.cost + 0.4
  end
end

coffee = Coffee.new
Sugar.new(Milk.new(coffee)).cost  # 2.6
Sugar.new(Sugar.new(coffee)).cost # 2.4
Sugar.new(Milk.new(coffee)).class # Sugar
Milk.new(coffee).origin           # NoMethodError