just1 :: (Show x) => Maybe x -> Maybe x -> Maybe x
just1 (Just x) (Just y) = error $ "expected only one of: "
  `mappend` show x `mappend` " " `mappend` show y
just1 :: (Show x) => Maybe x -> Maybe x -> Maybe x
just1 (Just x) (Just y) = error $ "expected only one of: "
  `mappend` show x `mappend` " " `mappend` show y
just1 (Just x) (Just y) = error $ "expected only one of: "
  `mappend` show x `mappend` " " `mappend` show y