anchor.diff:           tryPick/2,
            unfold/2,
            %where/2,
  -         %windowed/2,
  +         %windowed/2, % returns arrays
            zip/2,
            zip3/3,
            seq/1
  
positive.code:                 aggregate(Enum, {1, F(Item)}, Aggr)
          end,
      Total / Num.
  
  empty() ->
      {seq, {list, []}}.
  
negative.code:                                       [Item], Groups0),
              group_by(F, Groups, Seq)
      end.
  
  distinct_by(KeyF, Items, Seq0) ->
      case next(Seq0) of
          finished ->
              lists:reverse(Items);
          {Item, Seq} ->
              case lists:member(KeyF(Item), Items) of
                  true ->
                      distinct_by(KeyF, Items, Seq);
                  false ->
                      distinct_by(KeyF, [Item | Items], Seq)
              end
      end.
  
  
  
  next({seq, Enum}) ->
  