elements.rb

class CarElement
  # force subclasses to override the accept method
  def accept(visitor)
    raise NotImpelementedError.new
  end
end

class Wheel < CarElement
  include Visitable # now we have the 'accept' method

  attr_reader :name
  def initialize(name)
    @name = name
  end
end

class Engine < CarElement
  include Visitable # now we have the 'accept' method
end

class Body < CarElement
  include Visitable # now we have the 'accept' method
end