leap n = dv n 400 || dv n 4 && not (dv n 100) where dv x y = mod x y == 0
leap' n = False `unl` dv n 4 `unl` dv n 100 `unl` dv n 400
  where
    infixr `unl`
    unl a b = if b then not a else a
    dv x y = mod x y == 0