irred : [8]
irred = 0x1b
// endianness difference from Cryptol-1 test
xtimes a = (if a ! (width a - 1) then a' ^ irred else a')
  where a' = a << 1

check21 = [ xtimes y | y <- [ 1, 12, 192 ] ] == [ 2, 24, 155 ]