anchor.diff:      | Concat of RegexBuild list * ConcatFirstPos
       | Alt of RegexBuild list
       | Star of RegexBuild
  -    | Chars of RegexLeaf
  -    | End of RegexEnd
  +    | Leaf of RegexLeaf
   
   and internal RegexBuild = {
       Tree: RegexBuildTree
  
positive.code:             i'
      let leaves = ImmutableArray.CreateBuilder()
      let addLeaf x =
          leaves.Add(box x)
          x
  
      let desensitivizeCase chars =
  
negative.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 =
  