main :: Fay ()
main = do
  fr <- newFayRef (10 :: Int)
  readFayRef fr -- Should give 10.
  writeFayRef fr 20
  readFayRef fr -- Should give 20.
  modifyFayRef fr (*2)
  readFayRef fr -- Should give 40.
  -- The above was non-strict; here is the strict variant.
  modifyFayRef' fr (*2)
  readFayRef fr -- Should give 80.
  return ()