anchor.diff:      /// Compute the derivative of a regular vector
       /// with respect to the given symbol.
       let inline derivative symbol (regVec : RegularVector) : RegularVector =
  -        #if PARALLEL_FX
  -        Array.Parallel.map (Regex.Derivative symbol) regVec
  -        #else
           Array.map (Regex.Derivative symbol) regVec
  -        #endif
   
       /// Determines if the regular vector is nullable,
       /// i.e., it accepts the empty string (epsilon).
  
positive.code:     /// The indices of the element expressions (if any)
      /// that accept the empty string (epsilon).
      let acceptingElements (regVec : RegularVector) =
          /// The indices of the expressions accepting the empty string.
          let mutable accepting = Set.empty
  
          let len = Array.length regVec
          for i = 0 to len - 1 do
              if Regex.IsNullable regVec.[i] then
                  accepting <- Set.add i accepting
  
          // Return the computed set of indices.
          accepting
  
      /// Determines if a regular vector is an empty vector. Note that an
      /// empty regular vector is *not* the same thing as an empty array.
  
negative.code:             return Negate r'
          | CharacterSet charSet ->
              return
                  if CharSet.contains wrtSymbol charSet then
                      Epsilon
                  else
                      Regex.empty
          }
  
      /// Computes the derivative of a Regex with respect to a specified symbol.
  