anchor.diff:      let charSetFull = set [char 0 .. char UInt16.MaxValue]
       let isCharSetFull(x: char Set) = x.Count = int UInt16.MaxValue
       let isCharSetHalfFull(x: char Set) = x.Count > int UInt16.MaxValue / 2
  +
  +    let regexEmpty = Regex.Concat []
  +    let regexAny = Regex.AllButChars Set.empty
   
   type internal RegexStringHolder(regex) =
       let lockObj = obj()
  
positive.code:     /// The range is closed.
      member x.Between from upTo =
          if from > upTo then
              invalidArg "upTo" "'upTo' must be bigger or equal to 'from'"
          Regex.Join(x.Repeat from, x.Optional().Repeat <| upTo - from)
      /// Returns a regex that recognizes at least a number of
      /// consecutive strings that are recognized by the given regex.
  
negative.code:         str
          |> Seq.map Regex.Literal
          |> List.ofSeq
          |> function | [x] -> x | x -> Concat x
      /// Returns a regex that only recognizes a single literal character.
      static member Literal c = Set.singleton c |> Chars
      /// Returns a regex that recognizes only one character
  