anchor.diff:              end;
           {Item, Current} ->
               {Item, {collect, F, Current, Enum0}}
  -    end.
  -
  +    end;
  +
  +next({unfold, Gen, State0}) ->
  +    case Gen(State0) of
  +        undefined ->
  +            finished;
  +        {Item, State} ->
  +            {Item, {unfold, Gen, State}}
  +    end.
   
   do_filter(P, Enum0) ->
       case next(Enum0) of
  
positive.code: singleton(Item) ->
      {seq, {list, [Item]}}.
  
  skip(Num, Seq) ->
      {seq, {skip, Num, seq(Seq)}}.
  
  
negative.code:          ofList/1,
           pairwise/1,
           pick/2,
           %readonly/1, % operates on mutable data structures
           reduce/2,
           scan/3,
           singleton/1,
           skip/2,
           skipWhile/2,
  