code: def alias(self, alias):
          
          if alias is FIELDNAME:
              if not self._field:
                  raise ValueError("Cannot use FIELDNAME alias with no field")
              # Chop off initial '@'
              alias = self._field[1:]
          self._alias = alias
          return self
docs: Set the alias for this reducer.
  
          ### Parameters
  
          - **alias**: The value of the alias for this reducer. If this is the
              special value `aggregation.FIELDNAME` then this reducer will be
              aliased using the same name as the field upon which it operates.
              Note that using `FIELDNAME` is only possible on reducers which
              operate on a single field value.
  
          This method returns the `Reducer` object making it suitable for
          chaining.