divBy v (x:xs) =
  case divBy v xs of
       Nothing -> Nothing
       Just rxs -> Just ((v `div` x) : rxs)
divBy v (x:xs) =
  case divBy v xs of
       Nothing -> Nothing
       Just rxs -> Just ((v `div` x) : rxs)
divBy v (x:xs) =
  case divBy v xs of
       Nothing -> Nothing
       Just rxs -> Just ((v `div` x) : rxs)