anchor.diff:  let ofSeq (xs: 'a seq): 'a list =
       Seq.foldBack(fun x acc -> x::acc) xs []
   
  +let concat (lists : #seq<'a list>) =
  +    Seq.foldBack (List.foldBack (fun x acc -> x::acc)) lists []
  +
   let slice (lower: int option) (upper: int option) (xs: 'T list) =
       let lower = defaultArg lower -1
       let upper = defaultArg upper -1
  
positive.code:         |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
  
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 ->
  