code: def optionally(self, entity_type, attribute_name=None):
          
          if not attribute_name:
              attribute_name = entity_type
          self.optional += [(entity_type, attribute_name)]
          return self
docs: Parsed intents from this parser can optionally include an entity of the provided type.
  
          Args:
              entity_type(str): an entity type
              attribute_name(str): the name of the attribute on the parsed intent. Defaults to match entity_type.
  
          Returns:
              self: to continue modifications.