anchor.diff:          |None -> reverse acc
           |Some (x,state) -> unfoldInner (x::acc) state
       unfoldInner [] state
  +
   let rec foldIndexed3Aux f i acc bs cs ds =
      match bs, cs, ds with
      | [], [], [] -> acc
  
positive.code: let ofSeq (xs: 'a seq): 'a list =
      Seq.foldBack(fun x acc -> x::acc) xs []
  
  let slice (lower: int option) (upper: int option) (xs: 'T list) =
      let lower = defaultArg lower -1
      let upper = defaultArg upper -1
  
negative.code: 
  let item n xs =
     findIndexed (fun i _ -> n = i) xs
  
  let tryItem n xs =
     tryFindIndexed (fun i _ -> n = i) xs
  
  let filter f xs =
     foldBack (fun x acc ->
  