anchor.diff:  
   [<RequireQualifiedAccess>]
   module private RegexUtils =
  -    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
  
positive.code:             | _ -> Set.ofSeq xs
          if RegexUtils.isCharSetFull set then
              Regex.Empty
          elif RegexUtils.isCharSetHalfFull set then
              Chars <| RegexUtils.charSetFull - set
          else
              AllButChars set
      /// Returns a regex specified by a string.
  
negative.code:             | _ -> Set.ofSeq xs
          if set.IsEmpty then
              Regex.Empty
          else
              Chars set
      /// Returns a regex that recognizes any character,
  