     "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": "3.4.3"
 </s> add    "version": "3.5.0" </s> remove     "version": 2
 </s> add     "version": 3 </s> remove    "display_name": "Python 2",
 </s> add    "display_name": "Python 3", </s> remove    "name": "python2"
 </s> add    "name": "python3"
   "pygments_lexer": "ipython3",
   "version": "3.5.0"
     "file_extension": ".py",
     "mimetype": "text/x-python",
     "name": "python",
     "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.5.0"
   "pygments_lexer": "ipython3",
   "version": "3.5.0"
    }
   },
   "nbformat": 4,
   "nbformat_minor": 0
  }
keep keep keep keep replace replace keep keep keep keep keep
 Polish delete mid node challenge and solution

Update edge cases and tests. 
linked_lists/delete_mid/delete_mid_challenge.ipynb