        "Test: find general case with no matches\n",
        "Success: test_find\n",
        "\n",
        "Test: delete on an empty list\n",
        "Test: delete a NULL\n",
        "Test: delete general case with matches\n",
        "Test: delete general case with no matches\n",
        "Success: test_delete\n",
        "\n",
        "Test: len on an empty list\n",
 
</s> remove       "Test: find a NULL\n",
 </s> add       "Test: find a None\n", </s> remove       "Test: append a NULL\n",
 </s> add       "Test: append 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: NULL towers\n",
 </s> add       "Test: None towers\n", </s> remove     "* Delete a NULL\n",
 </s> add     "* Delete a None\n",
      "Test: delete a None\n",
        "Test: find general case with no matches\n",
        "Success: test_find\n",
        "\n",
        "Test: delete on an empty list\n",
      "Test: delete a None\n",
        "Test: delete general case with matches\n",
        "Test: delete general case with no matches\n",
        "Success: test_delete\n",
        "\n",
        "Test: len on an empty list\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