class FiberStack
  def initialize 
    @f = Fiber.new {irb}
    @f.resume #start irb loop
  end
 
  def empty?
    size == 0
  end
 
  def size
    @f.resume(:size).tap {@f.resume}
  end
  
  def push o
    @f.resume :push, o
  end
  
  def pop
    @f.resume :pop unless empty?
  end
  
  private
  def irb o = nil, size = 0
    popped = nil #initially nothing has been popped
    loop do
      cmd, val = Fiber.yield popped
      return o if cmd == :pop #pop command, return one level on call stack
      if cmd == :size
        Fiber.yield size 
      else
        popped = irb val, size+1 #recursive call to put val on call stack
      end
    end
  end
end