code: def group_by(self, fields, *reducers):
          
          group = Group(fields, reducers)
          self._groups.append(group)
  
          return self
docs: Specify by which fields to group the aggregation.
  
          ### Parameters
  
          - **fields**: Fields to group by. This can either be a single string,
              or a list of strings. both cases, the field should be specified as
              `@field`.
          - **reducers**: One or more reducers. Reducers may be found in the
              `aggregation` module.