anchor.diff:          | Negate Empty ->
               cont Any
           | Negate Any ->
  -            cont Empty        
  +            cont Empty
  +        | Negate Epsilon ->
  +            cont Empty
           | Negate (Character c) ->
                let anyMinusChar = CharSet.remove c charUniverse
                Regex.SimplifyCharacterSet anyMinusChar
  
positive.code:         
          Regex.CanonicalizeImpl regex universe id
  
      /// Computes a conservative approximation of the intersection of two sets of
      /// derivative classes. This is needed when computing the set of derivative
      /// classes for a compound regular expression ('And', 'Or', and 'Concat').
  
negative.code:     static member IsNullable (regex : Regex) =
          Regex.IsNullableImpl regex id
  
      /// Given a CharSet from a CharacterSet case, returns the simplest Regex
      /// representing the CharSet.
      static member inline private SimplifyCharacterSet (charSet : CharSet) =
  