anchor.diff:      [<CompiledName("CreateStar")>]
       let rec star (regex : Regex) : Regex =
           match regex with
  -        | Epsilon ->
  -            Epsilon
  -        | CharacterSet charSet
  -            when CharSet.isEmpty charSet ->
  +        | Epsilon
  +        | Empty ->
               Epsilon
           | Or (Epsilon, regex)
           | Or (regex, Epsilon)
  
positive.code:         | regex, Epsilon
          | Epsilon, regex ->
              return regex
          | CharacterSet charSet, _
          | _, CharacterSet charSet
              when CharSet.isEmpty charSet ->
              return empty
  
          // Nested Concat patterns should skew towards the right.
  
negative.code:         | _ ->
              false
  
      /// Partial active pattern which may be used to detect the 'empty' language.
      let inline private (|Empty|_|) regex =
          match regex with
          | CharacterSet charSet
              when CharSet.isEmpty charSet ->
              Some ()
          | _ ->
              None
  
      /// Returns a new regular expression which matches exactly one (1) instance of the specified character.
      [<CompiledName("OfCharacter")>]
      let inline ofChar c =
  