        "Test: insert_to_front general case\n",
        "Success: test_insert_to_front\n",
        "\n",
        "Test: append on an empty list\n",
        "Test: append a NULL\n",
        "Test: append general case\n",
        "Success: test_append\n",
        "\n",
        "Test: find on an empty list\n",
        "Test: find a NULL\n",
 
</s> remove       "Test: find a NULL\n",
 </s> add       "Test: find a None\n", </s> remove       "Test: insert_to_front on a NULL\n",
 </s> add       "Test: insert_to_front on a None\n", </s> remove       "Test: delete a NULL\n",
 </s> add       "Test: delete a None\n", </s> remove       "Test: NULL towers\n",
 </s> add       "Test: None towers\n", </s> remove     "        print('Test: append a NULL')\n",
 </s> add     "        print('Test: append a None')\n",
      "Test: append a None\n",
        "Test: insert_to_front general case\n",
        "Success: test_insert_to_front\n",
        "\n",
        "Test: append on an empty list\n",
      "Test: append a None\n",
        "Test: append general case\n",
        "Success: test_append\n",
        "\n",
        "Test: find on an empty list\n",
        "Test: find a NULL\n",
keep keep keep keep replace keep keep keep keep keep
 Changed discussions of NULL to None to be more Pythonic. 
linked_lists/linked_list/linked_list_solution.ipynb