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class Goodie
  attr_accessor :price

  def initialize(price)
    @price = price
  end

  def accept(visitor)
    visitor.visit(self)
  end
end

class TaxManVisitor
  def initialize(rate)
    @rate = rate / 100.0
  end

  def visit(obj)
    if obj.respond_to? :price
      obj.instance_eval { class << self; attr_accessor :tax; end }
      obj.tax = obj.price * @rate
    end
  end
end

x = Goodie.new(25)
puts "price is #{x.price}"
x.accept TaxManVisitor.new(9)
puts "tax is #{x.tax}"