code: def link(self, bucket='_', tag='_', keep=False):
          
          self._phases.append(RiakLinkPhase(bucket, tag, keep))
          return self
docs: Add a link phase to the map/reduce operation.
  
          :param bucket: Bucket name (default '_', which means all
              buckets)
          :type bucket: string
          :param tag:  Tag (default '_', which means any tag)
          :type tag: string
          :param keep: Flag whether to keep results from this stage in
            the map/reduce. (default False, unless this is the last step
            in the phase)
          :type keep: boolean
          :rtype: :class:`RiakMapReduce`