code: def note_deletion(self, key):
          
          # If we'rew deleting a key we previously added, then there is no diff
          if key in self._added:
              self._added.remove(key)
          else:
              # If the deleted key was previously changed, use the original value
              if key in self._previous:
                  self._deleted[key] = self._previous[key]
                  del self._previous[key]
              else:
                  self._deleted[key] = self._instance[key]
docs: Notes the deletion of a field.