anchor.diff:  next({skip, Num, Enum0}) ->
       case next(Enum0) of
           finished ->
  -            finished;
  +            throw_invalid_op_exn();
           {_SkippedItem, Enum} ->
               next({skip, Num-1, Enum})
       end;
  
positive.code: skip_test() ->
      [3] = toList(skip(2, [1,2,3])),
      [3] = toList(skipWhile(fun(I) -> I < 3 end, [1,2,3])),
      ok.
  -endif.
  
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,
  