How to I make private class constants in Ruby

In Ruby how does one create a private class constant? (i.e one that is visible inside the class but not outside)

class Person
  SECRET='xxx' # How to make class private??

  def show_secret
    puts "Secret: #{SECRET}"
  end
end

Person.new.show_secret
puts Person::SECRET # I'd like this to fail

You can also change your constant into a class method:

def self.secret
  'xxx'
end

private_class_method :secret

This makes it accessible within all instances of the class, but not outside.