Adding xorish operations to Data.Bits.Lens

xorBitsLens.hs

-- http://en.wikipedia.org/wiki/Rijndael_S-box

affineX' :: Word8 -> Word8 -> Word8
affineX' c b = b &~ forM_ [0..7] twiddleBits
  where
  twiddleBits :: Int -> StateT Word8 Identity ()
  twiddleBits j = do
    bitAt j    .= f b 4
    bitAt j .^|.= f b 5
    bitAt j .^|.= f b 6
    bitAt j .^|.= f b 7
    bitAt j .^|.= f c 0
      where
      f n o = n ^. bitAt (mod (o + j) 8)