class ImmutableStack
       
def pop()
    [@head, @tail]
 end
 
def push(element)
    ImmutableStack.new(element, self)
 end
 
def self.empty()
    @@empty ||= self.new(nil, nil)
 end
 
def each()
    yield(head)
t = tail
while t.!=(ImmutableStack.empty) do
  h, t = t.pop
  yield(h)
end

 end
 
def initialize(h,t)
    @head = h
@tail = t
self.freeze

 end

 private
attr_reader :head
      attr_reader :tail
      
    
    
      end