code: def add_index(self, field, value):
          
          if field[-4:] not in ("_bin", "_int"):
              raise RiakError("Riak 2i fields must end with either '_bin'"
                              " or '_int'.")
  
          self.indexes.add((field, value))
  
          return self._robject
docs: add_index(field, value)
  
          Tag this object with the specified field/value pair for
          indexing.
  
          :param field: The index field.
          :type field: string
          :param value: The index value.
          :type value: string or integer
          :rtype: :class:`RiakObject <riak.riak_object.RiakObject>`