testList :: RP s (SRef s (RPList s Int))
testList = do
  tail <- newSRef Nil
  c1   <- newSRef $ Cons (- 1) tail
  c2   <- newSRef $ Cons 1     c1
  newSRef $ Cons 1 c2
testList :: RP s (SRef s (RPList s Int))
testList = do
  tail <- newSRef Nil
  c1   <- newSRef $ Cons (- 1) tail
  c2   <- newSRef $ Cons 1     c1
  newSRef $ Cons 1 c2
testList = do
  tail <- newSRef Nil
  c1   <- newSRef $ Cons (- 1) tail
  c2   <- newSRef $ Cons 1     c1
  newSRef $ Cons 1 c2