{-# LANGUAGE Haskell2010 #-}
import Control.Monad.ST
import Data.STRef

-- Make sure that the realWord# token used by runST (actually by runSTRep) is
-- not inlined, which would share the call to `newMutVar#`, and suddenly there
-- would be only one MutVar

main =
    let f () = runST $ do
            ref <- newSTRef 0
            modifySTRef ref (+1)
            readSTRef ref
    in print (f () + f ())