     "execution_count": 14,
     "metadata": {
      "collapsed": false
     },
     "outputs": [
      {
       "name": "stdout",
       "output_type": "stream",
       "text": [
        "Dequeue on empty stack\n",
        "None\n",
        "Enqueue on empty stack\n",
        "Enqueue on non-empty stack\n",
        "Multiple enqueue in a row\n",
        "Dequeue on non-empty stack\n",
        "Dequeue after an enqueue\n",
        "0\n",
        "Multiple dequeue in a row\n",
        "1\n",
        "2\n",
        "Enqueue after a dequeue\n",
        "5\n",
        "None\n"
       ]
      }
     ],
     "source": [
      "print('Dequeue on empty stack')\n",
      "queue = QueueFromStacks()\n",
      "print(queue.dequeue())\n",
      "print('Enqueue on empty stack')\n",
 
</s> remove   },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
 </s> add  </s> remove    "execution_count": 14,
 </s> add    "execution_count": null, </s> remove    "execution_count": 8,
 </s> add    "execution_count": null, </s> remove    "execution_count": 3,
 </s> add    "execution_count": null, </s> remove     "    def is_stack_empty(self, stack):\n",
    "        return stack.peek() is None\n",
    "\n",
 </s> add 
   "outputs": [],
     "execution_count": 14,
     "metadata": {
      "collapsed": false
     },
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
   "outputs": [],
     "source": [
      "print('Dequeue on empty stack')\n",
      "queue = QueueFromStacks()\n",
      "print(queue.dequeue())\n",
      "print('Enqueue on empty stack')\n",
keep keep keep keep replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace replace keep keep keep keep keep
 Updated problem solution to use stack's is_empty() method rather than a similar method that used to be in QueueFromStacks. 
stacks-queues/queue-from-stacks.ipynb