anchor.diff:          |> fst
       | Alt xs -> xs |> Seq.map (fun x -> x.FirstPos.Value) |> Set.unionMany
       | Star x -> x.FirstPos.Value
  -    | Chars x -> Set.singleton x.Index
  -    | End x -> Set.singleton x.Index
  +    | Leaf x -> Set.singleton x.Index
   
   let private fLastPos =
       function
  
positive.code:         regexes
          |> List.map (fun (regex, acceptSymbol) -> createRegexBuildSingle regex acceptSymbol)
          |> (function | [x] -> x | x -> makeLazy<| Alt x)
      
      theTree, leaves.ToImmutable() |> RegexBuildLeaves
  
  let internal calculateFollowPos leafCount regex =
  
negative.code:                         chars
                      else
                          desensitivizeCase chars
                  RegexLeaf.Chars(fIndex(), chars) |> addLeaf |> Leaf
              |> makeLazy
          let regexBuild = impl regex
          let endLeaf = RegexLeaf.End(fIndex(), acceptSymbol) |> addLeaf |> Leaf |> makeLazy
          match regexBuild.Tree with
          | Concat (xs, _) -> xs @ [endLeaf]
          | _ -> regexBuild :: [endLeaf]
  