incMVar :: MVar Int -> IO ()
incMVar mv = do
  i <- takeMVar mv
  putMVar mv (i + 1)
incMVar :: MVar Int -> IO ()
incMVar mv = do
  i <- takeMVar mv
  putMVar mv (i + 1)
incMVar mv = do
  i <- takeMVar mv
  putMVar mv (i + 1)