code: def new(self, val):
          
          if len(self.things) >= self.max_things:
              raise LimitationError('too many things')
          self.things.add(val)
          return val
docs: Add a new value to me.
  
          Args:
              val (LispVal): The value to be added.
  
          Returns:
              LispVal: The added value.
  
          Raises:
              ~parthial.errs.LimitationError: If I already contain the maximum
                  number of elements.