parity xs = ys!0
  where ys = [False] # [y ^ x | x <- xs | y <- ys ]

foldl1 f xs = ys!0
  where ys = [xs@0] # [ f y x | y <- ys | x <- tail xs ]

par = foldl1 (^)

parOK : {n} [n+1] -> Bit
property parOK x = par x == parity x