von Neumann Integer.

ni.rb

class Fixnum
  def to_ss
    if self == 0
      ""
    elsif self == 1
      "{}"
    else
      (self - 1).to_ss + ',' + (self - 1).to_s
    end
  end

  def to_s
    "{" + self.to_ss + "}"
  end
end

puts 0  # => {}
puts 1  # => {{}}
puts 2  # => {{},{{}}}
puts 3  # => {{},{{}},{{},{{}}}}

puts -1 # => stack level too deep (SystemStackError)

Web連載「数学ガールの秘密ノート」第151回　3を作ろう（前編）
https://cakes.mu/posts/12710