mutateVal :: Env -> Index -> AtomoVal -> IOThrowsError AtomoVal
mutateVal e n v = do ref <- getRef e n
                     liftIO $ writeIORef ref v
                     return v
mutateVal :: Env -> Index -> AtomoVal -> IOThrowsError AtomoVal
mutateVal e n v = do ref <- getRef e n
                     liftIO $ writeIORef ref v
                     return v
mutateVal e n v = do ref <- getRef e n
                     liftIO $ writeIORef ref v
                     return v