part5.rb

class Class
  def attr_accessor_with_history(attr_name)
    attr_name = attr_name.to_s
    attr_reader attr_name
    attr_reader attr_name+"_history"
    define_method("#{attr_name}=") do |value|
      prev = instance_variable_get "@#{attr_name}_history"
      prev = [nil] if prev.nil?
      prev << value
      instance_variable_set "@#{attr_name}_history", prev
      instance_variable_set "@#{attr_name}", value
    end
  end
end

class Foo
    attr_accessor_with_history :bar
end