context :ImmutableStack do
  role :head do
  end
  role :tail do
  end
  pop do
    [@head, @tail]
  end
  push do |element|
    ImmutableStack.new element, self
  end

  empty self do
    @@empty ||= self.new(nil, nil)
  end

  each do
    yield head
    t = tail
    while t != ImmutableStack::empty do
      h, t = t.pop
      yield h
    end
  end

  initialize do |h, t|
    @head = h
    @tail = t
    self.freeze
  end
end