     "file_extension": ".py",
     "mimetype": "text/x-python",
     "name": "python",
     "nbconvert_exporter": "python",
     "pygments_lexer": "ipython2",
     "version": "2.7.10"
    }
   },
   "nbformat": 4,
   "nbformat_minor": 0
  }
 
</s> remove     "version": 2
 </s> add     "version": 3 </s> remove    "display_name": "Python 2",
 </s> add    "display_name": "Python 3", </s> remove       "Test: Pop on a one element stack to destroy it\n",
 </s> add       "Test: Pop on a stack to destroy it\n", </s> remove     "            if num_stacks == 0:\n",
    "                self.last_stack = None\n",
    "            else:\n",
    "                self.last_stack = self.stacks[num_stacks-1]\n",
    "        else:\n",
    "            data = self.last_stack.pop()\n",
 </s> add 
   "pygments_lexer": "ipython3",
   "version": "3.4.3"
     "file_extension": ".py",
     "mimetype": "text/x-python",
     "name": "python",
     "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.4.3"
   "pygments_lexer": "ipython3",
   "version": "3.4.3"
    }
   },
   "nbformat": 4,
   "nbformat_minor": 0
  }
keep keep keep keep replace replace keep keep keep keep keep
 Polish set of stacks solution 
stacks_queues/set_of_stacks/set_of_stacks_solution.ipynb