anchor.diff:          | Epsilon ->
               return DerivativeClasses.Universe
           | Any ->
  -            return
  -                { DerivativeClasses.Universe
  -                    with HasEmptySet = true; }
  +            return DerivativeClasses.UniversePlusEmptySet
           | CharacterSet charSet ->
               return
                   { HasEmptySet = false;
  
positive.code:                     Regex.epsilon
                  else
                      Regex.empty
          }
  
      /// Computes the derivative of a Regex with respect to a specified symbol.
      static member Derivative symbol regex =
          Regex.DerivativeImpl symbol regex id
  
      /// Computes an approximate set of derivative classes for the specified Regex.
      static member private DerivativeClassesImpl regex =
  
negative.code:     /// Determines if a specified Regex is 'nullable',
      /// i.e., it accepts the empty string (epsilon).
      static member IsNullable (regex : Regex) =
          // OPTIMIZATION : Immediately return the result for some patterns instead of calling
          // the continuation-based method -- this eliminates the overhead of creating/calling
          // the continuations for some very common cases.
          match regex with
          | Epsilon
          | Star _ ->
              true
          | Any
          | CharacterSet _ ->
              false
          | _ ->
              Regex.IsNullableImpl regex id
  
  
  /// 'Smart' constructors for Regex.
  