code: def derive_field_name(self, field_name):
          
          cls = type(self)
          # We use ordinals to avoid thunk materialization.
          return cls(
              self[0],
              self[1],
              self[2],
              field_name,
              self[4],
              self[5]
          )
docs: Derives a new event from this one setting the ``field_name`` attribute.
  
          Args:
              field_name (Union[amazon.ion.symbols.SymbolToken, unicode]): The field name to set.
          Returns:
              IonEvent: The newly generated event.