anchor.diff:      find_internal(seq(Seq), fun (I) -> I =:= Item end) =/= undefined.
   
   tail(Seq) ->
  -    skip(1, Seq).
  +    {seq, {tail, first, seq(Seq)}}.
   
   take(Num, Seq) ->
       {seq, {take, Num, seq(Seq)}}.
  
positive.code: singleton(Item) ->
      {seq, {list, [Item]}}.
  
  skip(Num, Seq) ->
      {seq, {skip, Num, seq(Seq)}}.
  
  
negative.code:          tryFind/2,
           tryFindIndex/2,
           tryPick/2,
           unfold/2,
           %where/2,
           %windowed/2,
           %zip/2,
  