code: def note_addition(self, key, value):
          
          # If we're adding a field we previously deleted, remove the deleted note.
          if key in self._deleted:
              # If the key we're adding back has a different value, then it's a change
              if value != self._deleted[key]:
                  self._previous[key] = self._deleted[key]
              del self._deleted[key]
          else:
              self._added.append(key)
docs: Updates the change state to reflect the addition of a field. Detects previous
          changes and deletions of the field and acts accordingly.