     "metadata": {},
     "source": [
      "## Test Cases\n",
      "\n",
      "* NULL tower(s)\n",
      "* 0 disks\n",
      "* 1 disk\n",
      "* 2 or more disks"
     ]
    },
 
</s> remove     "* NULL tower(s)\n",
 </s> add     "* None tower(s)\n", </s> remove       "Test: NULL towers\n",
 </s> add       "Test: None towers\n", </s> remove     "* Insert a NULL\n",
 </s> add     "* Insert a None\n", </s> remove     "* Insert a NULL\n",
 </s> add     "* Insert a None\n", </s> remove     "* Append a NULL\n",
 </s> add     "* Append a None\n",
    "* None tower(s)\n",
     "metadata": {},
     "source": [
      "## Test Cases\n",
      "\n",
    "* None tower(s)\n",
      "* 0 disks\n",
      "* 1 disk\n",
      "* 2 or more disks"
     ]
    },
keep keep keep keep replace keep keep keep keep keep
 Changed discussions of NULL to None to be more Pythonic. 
stacks_queues/hanoi/hanoi_challenge.ipynb