class Cons
  def car; @e[0]; end
  def cdr; @e[1]; end

  def initialize(car,cdr)
    @e = [car,cdr]
  end

  def inspect(firstp=true)
    if cdr.class == Cons
      if firstp
        "(#{car.inspect} #{cdr.inspect(false)})"
      else
        "#{car.inspect} #{cdr.inspect(false)}"
      end
    elsif cdr.nil?
      if firstp
        "(#{car.inspect})"
      else
        "#{car.inspect}"
      end
    elsif firstp
      "(#{car.inspect} . #{cdr.inspect})"
    else
      "#{car.inspect} . #{cdr.inspect}"
    end
  end

  def self.list(*args)
    args.reverse.inject(nil){|acc,e| Cons.new(e, acc)}
  end
end

if __FILE__ == $0
  p Cons.new(1,nil)
  p Cons.new(1,2)
  p Cons.new(1, Cons.new(2,3))
  p Cons::list()
  p Cons::list(1,2,3)
  p Cons::list(1,2,3,4,5)
  p Cons::list(1,Cons::list(2,3),4)
end