code: def _update_field(self, action, field, value, max_tries, tries=0):
          
          # Refresh our view of the document.
          self.fetch()
  
          # Update the field.
          action(self, field, value)
  
          # Attempt to save, retrying conflicts up to max_tries.
          try:
              self.save()
          except requests.HTTPError as ex:
              if tries < max_tries and ex.response.status_code == 409:
                  self._update_field(
                      action, field, value, max_tries, tries=tries+1)
              else:
                  raise
docs: Private update_field method. Wrapped by Document.update_field.
          Tracks a "tries" var to help limit recursion.